Introduction to Brake Systems – Study Guide
Contents
1. it pulls cables that are attached to actuator levers and struts inside the brake drum These actuator levers and struts mechanically apply the brakes by pushing both brake shoes outward into the drum fig 52 Shoe strut Rear Cable Latch Release RH P4 mit Parking Brake Pedal Front Cable Adjuster screw fig 51 Typical Parking Brake System fig 52 Parking Brake Strut and Actuator Lever Duo Servo Disc Brakes with Integral Parking Brake Many vehicles that are equipped with rear disc brakes require a regular application of the parking brake to keep the rear disc brakes in proper adjustment Unlike front disc brakes rear disc brakes on these vehicles are not self adjusting The two 2 most common types of caliper actuated parking brakes are the e Screw and nut e Ball and ramp Screw and Nut Parking Brake When the parking brake is applied on a rear disc equipped vehicle fig 53 the following occurs 1 The cable actuated parking brake lever rotates an actuator screw 2 The actuator screw unthreads on a nut inside the piston 3 As the screw turns it moves the nut outward by pressing against a cone inside the piston 4 The piston applies the inboard pad against the rotor The movement of the piston also causes the caliper assembly to slide and apply the outboard pad 5 An adjuster spring inside the nut and cone rotates the nut outward when the parking brakes are released to provide self adjustme2. Caliper body or housing Internal hydraulic passages One or more pistons Piston seals Dust boots Bleeder screw Inboard and outboard disc pads Mounting bolts The most common types of disc brakes are the floating caliper and the sliding caliper Both the floating and sliding calipers operate identically and the only difference is in the mounting Specifically floating calipers slide on mounting bolts and bushings fig 26 while sliding calipers operate on machined guides and bushings fig 27 Re Mounting bolts EO a Bleeder Bleeder valve Valve cap Caliper housing gee 5 Prai Seal Piston Boot Inboard pad Guiboaard pad fig 26 Floating Caliper on Mounting Bolts and Bushings 11 2004 Melior Inc Introduction to Brake Systems Study Guide Cali Alper ways _ Caliper support Y anchor plate Caliper housing Piston Epot Seal Inboard pad Outboard pad Key relaining screw AURPE Mery Antirattle spring Support spring fig 27 Machined Guide Sliding Caliper Disc Brake Rotors The rotor or rotor hub assembly is attached to the wheel and provides the friction surface that the disc brake pads clamp against to slow and stop a vehicle Rotors must be machined and maintained to very close tolerances Those that are warped excessive lateral runout or have excessive thickness variation different thicknesses around the rotor can cause vibrations and shutte
3. Cylinder Brakes 5 N N To Rear Drum Brakes Brakes Released VEE NW 0 psi 4 w WW Metering Valve Stem To Front Disc Brakes Open fig 14 Brakes Released below 25 psi metering valve stem is open Proportioning Valve Under heavy braking conditions rear drum brakes are more susceptible to premature lock up than the front disc brakes Part of the reason is that rapid braking forces tend to pitch the vehicle forward which in turn reduces the weight on the rear wheels Reducing the weight on the rear wheels increases the likelihood of lock up Proportioning valves are therefore used in the rear hydraulic circuit s to help prevent this sort of premature lock up e During normal braking or when the brakes are first applied the proportioning valve is open and has no effect Fluid enters the valve through the end with the smaller piston area fig 15 passes through the small bore and exits to the rear brakes e Notice that the outlet end of the valve piston has a larger surface area than the inlet end of the valve When fluid pressure rises rapidly in the valve under hard braking it exerts a greater force on the larger outlet piston than it does on the smaller inlet piston This action moves the valve against spring pressure toward the inlet and closes the center valve With the valve closed pressure to the rear brakes is blocked fig 16 e As the inlet pressure from the master cylinder continues to rise it e
4. Piston Seal Piston Seal Piston Seal fig 9 Compensating Ports fig 10 Bypass Ports During brake release the following occurs e Strong springs in the master cylinder force the pistons back to the at rest position faster than the brake fluid can return through the hydraulic channels The pistons must return rapidly so they can be ready for another forward stroke if necessary This rapid piston return movement could create a vacuum in the master cylinder high pressure chambers which would delay brake release e The bypass ports allow brake fluid from the reservoir to fill the low pressure piston chambers e Brake fluid from the low pressure chambers then passes through holes in the pistons and bypasses the piston lip seals The pistons can then return without any dragging fig 11 Since this return action causes additional fluid to be moved to the front of the piston it results in an excess amount of fluid being present there as even more fluid returns from the calipers and wheel cylinders This excess fluid is easily returned to the reservoir through the now open compensating ports Note Piston dragging can also occur if the seals are installed backward Compensating Port Compensating Port Bypass Port Bypass Port fig 11 Master Cylinder Return Operation applied left releasing right Residual Check Valve drum brakes only e Included in the master cylinder e Located in the ports where the brake
5. application the rear brakes can lock up resulting in a skid and loss of vehicle control e Proportioning valve s are used to prevent rear brake lockup by limiting hydraulic pressure to the rear brakes during heavy braking e The metering valve and the proportioning valve are often housed in a single unit called a combination valve in many rear wheel drive vehicles equipped with front disc and rear drum brakes e Most vehicles are equipped with some form of pressure differential valve and switch which will activate a dashboard warning light if pressure is lost in either of the hydraulic channels This switch is typically located in a combination valve or on the master cylinder Metering Valve As a result of their design rear drum brake shoes must move a greater distance to apply as compared to disc brake pads If the same pressure were applied to both the front disc and rear drum brakes at the same time the front discs would catch much sooner than the rears and cause the vehicle to be thrown forward Metering valves are therefore used to compensate for this condition by blocking fluid pressure to the front disc brakes until the rear shoes have had time to make contact with the drums e As the brakes are first applied fluid pressure rises above a calibrated value approximately 25 psi figure 12 which closes the metering valve stem and blocks the fluid pressure from reaching the front disc brakes However fluid pressure is still appli
6. are sometimes housed together in a single unit called a combination valve fig 18 Combination valves are used only on Front Rear split brake systems Metering valve Valve Switch terminal Front imet pari E Froni outlet port s E Diaphragm a Alli Ti B bs Mi as bas F F ph Sate Pressure differential MENA pi 5 ma Propertioning Valve Rear inlet port Proportioning nut 1 Rear outlet j i port iy T_T Boot e EE tei s U il z Metering valve Fi yslem F l el Metering valve Seal Front outlet part i Propartioning piston B fede i Dias Sse iam a l Valve stem Stop plate 2004 Melior Inc Introduction to Brake Systems Study Guide Power Assist System Most modern vehicles are equipped with a power assist boost system to aid the driver when applying the brakes The two most common types of assist systems are vacuum assist and hydraulic assist Vacuum Booster The two 2 types of vacuum boosters used on modern vehicles are the single diaphragm fig 19 and the tandem diaphragm or dual diaphragm booster fig 20 Both booster types operate similarly but the tandem diaphragm booster is smaller in diameter and is used on vehicles where space is critical Manifeld Vaccum Hose Fri
7. brake pedal several times until the pedal becomes hard Maintain moderate foot pressure and start engine Does the pedal fall slightly Run engine for 15 seconds then turn engine off for 15 seconds Pump pedal several times to check for booster vacuum reserve Available assist _ 28 Introduction to Brake Systems Study Guide Check pedal linkage and hardware for binding rubbing and friction Correct as necessary Brake fluid leak Possible external leak Repair leak Possible internal hydraulic leak in master cylinder Repair master cylinder Possible fluid leak or air in The system Repair leak Bleed system Repair power brake booster Check Vacuum source Check vacuum check valve Check hydraulic assist Check pedal for proper operation 2004 Melior Inc Introduction to Brake Systems Study Guide Test Drive Drive vehicle at low speed under 20 mph Brake moderately to a stop e Unusual pedal effort hard spongy grabby e Noises from the brakes growl squeal scrape e Vehicle direction change pull See symptom chart in the appropriate service manual Note condition and proceed Drive vehicle at higher speed up to 40 mph Press the brake pedal with moderate force e Brake pedal pulsation e Vehicle or steering wheel vibration e Noise from the brakes e Direction change pull e Brake warning light on Repeat procedure using the parking brake mechanism wh
8. spring will expand and push the fluid into the booster for braking assist Accumulators can typically provide sufficient emergency hydraulic pressure for two or three 2 or 3 brake applications if power steering pressure is lost fig 24 Accumulator piston Plunger and check ball HUL Sj aos ag TEES ZN 4 a Spool valve i i a a4 Bova 2 LELELELI Pee e am VMTU ANWR LIF jaras eann 1 d Accumulator piston fig 24 Hydraulic Booster with Accumulator 10 2004 Melior Inc Disc Brakes Disc brakes are used on the front of all modern vehicles while some have both front and rear disc brakes The advantages of disc brakes over drum brakes include e Better fade resistance e Reduced pulling and grabbing e Self adjustment capability Disc brakes consist of the following components fig 25 Rotor Hub Caliper assembly Brake pads Mounting bolts Disc Brake Caliper The disc brake caliper converts hydraulic pressure from the master cylinder to a mechanical force that pushes the brake pads against the rotor The caliper body is a U shaped casting mounted over the rotor and is typically made of iron or aluminum All calipers regardless of design contain these major parts fig 26 Introduction to Brake Systems Study Guide Mounting Rotor Bolt Caliper i Hub eer Mounting Brake Bolt Pad fig 25 Principle Parts of a Disc Brake
9. Introduction to Brake Systems Study Guide 2004 Melior Inc Introduction Everybody knows that when you press your foot on the brake pedal the vehicle is supposed to stop But how does the pressure from your foot get to the wheels with enough force to stop a heavy vehicle In the following sections we will study the systems and components required to allow brakes to work effectively Course Objectives Upon completion of this course technicians should understand and be able to apply their knowledge of Brake functions and components Split hydraulic systems Master cylinder operations Balance control systems Power brake booster systems Disc brake operation Micrometer reading Drum brake operation Brake fluids Brake bleeding operations Brake lines and hoses Basic diagnosis Using the Job Sheets As you proceed through the online module on some pages you will find links that will open a window with a printable procedure or job sheet containing hands on lab activities based on the NATEF standards related to the content you are studying When you come upon a procedure or job sheet link click on it and print the job sheet for completion in the shop See your instructor for guidance in completing the job sheets Some jobs sheets will require supplemental materials such as a vehicle service manual equipment manual or other references Brake System Functions Automotive brakes are designed to slow and stop a vehicle by transforming kin
10. e disengaged if a brake warning light comes on with or without applying the service brakes it could indicate a hydraulic failure in the system WARNING Repair the hydraulic system before driving the vehicle Detailed Visual Inspection Component Corrective Action Brake pipes and hoses Repair or replace as necessary Crimps or restrictions Parking brake cables Clean lubricate adjust or replace brake application or release Parking brake operation Proper operation Clean lubricate adjust or replace Wheels rotate parking brake Jas necessary engaged Wheels cannot be rotated without excessive drag parking brake disengaged Brake linings Excessive wear Replace Brake hardware and hold Damage wear or corrosion Replace downs Missing Components Brake Rotors Wear reduced thickness Replace Deep scoring or scratches Compare to specifications Thickness variation Machine or replace as indicated Lateral runout Excessive heat checking Brake Drums Wear excessive diameter Compare to service limit Deep scoring or scratches specifications Taper bell mouth Out of round Excessive heat checking PEPI ae 2004 Melior Inc Brake Pedal Checks Engine off Press brake pedal several times Pedal friction or noise Press brake pedal lightly for 15 seconds Repeat with heavy foot pressure Does the pedal fall Pump brake pedal several times Is Pedal travel within specification Brake booster check Press
11. e that enters the hydraulic system The three 3 brake fluids currently assigned DOT numbers are DOT 3 DOT 4 and DOT 5 DOT 3 and DOT 4 are polyalkylene glyco ether mixtures while DOT 5 is silicone based All domestic and most import car manufacturers specify and require DOT 3 brake fluid some _ imports require DOT 4 as it has a higher boiling Bra DUN point DOT 5 brake fluid is not currently used in untae CD a any domestic or import vehicles fig 46 fig 46 DOT 3 and DOT 4 Brake Fluids Precautions must always be observed when working with brake fluids e Brake fluid is toxic to the human body e Brake fluid can damage painted surfaces e Brake fluid contaminated with moisture dirt petroleum or other foreign material will damage the hydraulic system internally e Only denatured alcohol or other approved cleaners should be used when cleaning brake hydraulic parts e Use only fresh clean brake fluid never reuse old brake fluid e Never mix brake fluids with any other fluids including other types of brake fluid e g DOT 3 and DOT 4 Bleeding Brakes Any time a brake hydraulic system is opened to the atmosphere for repairs or due to a leak the system must be bled to remove the air Unlike brake fluid air is compressible and can cause a spongy brake pedal brake pull and ineffective brake application Master Cylinder Bleeding lt is always a good idea to bench bleed a master cylinder after servicing and before in
12. eased position return springs hold the bottoms of the shoes against the adjusting screw while the tops of the shoes are held against the anchor pin As the brake pedal is pressed the following occurs fig 38 Hydraulic pressure from the master cylinder to the wheel cylinder forces both wheel cylinder pistons outward to press the shoes against the drum As the brake shoes contact the rotating drum frictional force causes both shoes to rotate slightly This action causes the secondary shoe the one toward the rear of the vehicle to jam against the anchor pin and forces the wheel cylinder piston back into the wheel cylinder The rotating action of the primary brake shoe the front one causes the secondary shoe to wedge into the drum with a force that is greater than the just the hydraulic pressure would cause Because of the wedging action both shoes must be pulled away from the drum by the return springs when the brakes are released Additionally there are other springs that hold the brake shoes in place and return the adjuster arm after it actuates fig 38 _16 As a result of this design the secondary shoes must perform more of the braking than the 2004 Melior Inc Introduction to Brake Systems Study Guide primary shoes Therefore the secondary shoes usually wear more and are typically larger than the primary shoes As a general rule of thumb the heavier a drum brake equipped vehicle is the more likely it i
13. ed to the rear brakes which move the shoes out to contact the drums e Once the shoes begin to contact the drums the pressure in the rear brake system starts to rise dramatically After the pressure reaches a second calibrated value about 100 psi figure 13 the metering valve opens and begins to apply the front disc brakes e As the brakes are released and the system pressure again drops below 25 psi the valve stem reopens to allow fluid to return to the master cylinder fig 14 Notice also in figures 12 through 14 that there is a difference between the valve stem and the valve itself Each is operated by a separate spring and has a separate function Note The two pressure points 25 psi amp 100 psi in this example are calibrated based on the size and weight of a particular vehicle Metering valves are not universally interchangeable even though they may appear to be identical 2004 Melior Inc Introduction to Brake Systems Study Guide From Master From Master To Front Disc Cylinder To Front Disc Cylinder Brakes Brakes f is j 3 To Rear Drum To Rear Drum Brakes Brakes W Brakes Applied al over 100 psi AM Brakes Applied over 25 psi WA Metering Valve Stem f Metering Valve To Front Disc Brakes Closed To Front Disc Brakes Open fig 12 Brakes Applied over 25 psi fig 13 Brakes Applied over 100 psi metering valve stem closed metering valve open From Master To Front Disc
14. el brakes work together on a separate system RF LR RR LF fig 3 Diagonally split Hydraulic System fig 4 Front Rear Split Hydraulic System 2004 Melior Inc Introduction to Brake Systems Study Guide Hydraulic System An important hydraulic principle states that fluids do not compress or produce any measurable friction Also fluid pressure does not diminish when transferred within a closed system That means that if there is no leak in a system the pressure at the wheels will be the same as the pressure from the master cylinder A second hydraulic principle states that a relationship exists between e Force and piston area e Piston travel and piston area From the first principle if a master cylinder generates 500 psi it also transfers 500 psi to the pistons in each wheel cylinder remember that fluid pressure remains constant In the second principle fig 5 when pressure from a one square inch master cylinder piston exerts 500 psi on a wheel cylinder piston which also has one square inch surface area the wheel cylinder piston transfers 500 pounds of force to the brake shoe 500 psi x 1 in sq 500 Ibs However if the same one square inch master cylinder piston exerts 500 psi on a wheel cylinder piston that has a two square inch area the wheel cylinder piston will transfer 1 000 pounds of force to the brake lining 500 psi x 2 in sq 1 000 Ibs Additionally different piston sizes not only affect the amo
15. etic motion energy into heat energy As the brake linings contact the drums rotors they create friction which produces the heat energy The intensity of the heat is proportional to the vehicle speed the weight of the vehicle and the quickness of the stop Faster speeds heavier vehicles and quicker stops equal more heat Automotive brake systems can be broken down into several different sub systems fig 1 e Apply system Boost system aoa aam Hydraulic system Hydrauta Syan Wheel brakes Balance control system Warning system Balance Control System Warning System Apply System lo _ a _ _ Wheel Brakes r fig 1 Base Brake Systems 2004 Melior Inc Introduction to Brake Systems Study Guide Base brake components are the parts of the Brake Booster and Linkage brake system commonly found on all vehicles Piivi Brake Waming Light The term base brakes does not include either Master Rear Brake Drum Antilock or Traction Control systems Base fe R brake components include b e Brake pedal and linkage Power assist system Master cylinder hoses and lines Brake rotors and pads Brake drums and shoes Balance controls proportioning valve and metering valve if equipped e Brake pressure and other warning iistiting Valve Proportioning Valve Front Brake syste mS Caliper e Parking brake pedal and linkage fig 2 Base Brake Components Parking Brake Pedal And Linkage Brake Li
16. h 0 0076 mm between four measurements fig 30 may require that the rotor be refinished or replaced during brake service refer to vehicle service manual for specifications Lining Contact Area Micrometer Measuring for Rotor Thickness Variation Measure rotor in four or more locations fig 29 fig 30 Minimum thickness specification in millimeters Rotor Refinishing Rotors should be refinished only in cases of e Excessive lateral runout e Excessive thickness variation e Excessive surface scoring fig 31 Discard Dimension on a Rotor Additionally there are two specifications that must be observed when refinishing rotors e Discard specification This specification is usually stamped or cast into the rotor Rotors can be reused to this minimum thickness specification if the rotor is not refinished fig 31 e Minimum refinish specification This specification is found in the vehicle service manual and is the minimum thickness to which a rotor can be refinished The difference between the discard and refinish specifications is to allow for the wear that takes place as the new pads burnish or wear into the refinished rotor _13 2004 Melior Inc Reading a 0 to 1 Inch English Micrometer The major components of a micrometer are fig 32 Frame Spindle and Thimble Sleeve Anvil Ratchet Locknut The spindle and thimble are made together and are threaded into the sleeve When the thimb
17. hicles became commonplace most vehicles had rear wheel drive and front rear split brake systems at least after the advent of dual piston master cylinders In order to bleed the front brakes it was necessary to build up enough pressure in the rear brakes to open the metering valve and allow fluid to reach the fronts So technicians would bleed the right rear first since it is the longest brake line and then move to the left rear After the rears were clear of trapped air the fronts could be bled starting with the longest line right front This system is still applicable today for vehicles with front rear split systems However it does not apply with diagonally split systems mostly front wheel drive for two reasons 1 diagonally split systems do not have a metering valve and 2 the right rear and left rear brakes are on separate systems If the procedure above is used on a diagonally split system and the right rear brake is bled followed by the left rear then the left front brake portion which is only half bled right rear is done will tend to aerate or make bubbles in the fluid as the pedal is pressed to bleed the left rear Once you have bubbles in the fluid it can be extremely difficult to get the lines clear and you may have to wait for the air and the fluid to separate again before continuing So what is the proper bleeding order to use It depends on the vehicle If it is a front rear split system start with one of the rear brakes i
18. icrometer 11 375 to get our total diameter of 11 390 inches 11 375 0 015 11 390 inches To determine how much metal we can remove from this drum and still use it we simply subtract the measured diameter from the maximum refinish diameter 11 485 11 390 0 045 For this example a maximum of 0 045 inch or 45 1000 inch can be machined from the drum Some Brake Service Thoughts Always inspect and measure the brake drums when replacing brake linings or if any of the following symptoms occur Pulsation Brake fade Chatter Wheel drag Brakes too sensitive Spongy pedal Resurface drums if e Taper or out of roundness exceeds 0 006 inch 0 15 mm e Scoring exceeds 0 060 inch 1 52 mm Replace drums if e The maximum diameter reading equals or exceeds the discard dimension e The drum is under the discard dimension but refinishing would not leave at least 0 030 inch 0 76 mm allowance for wear 19 2004 Melior Inc Introduction to Brake Systems Study Guide Brake Fluid The specifications for all automotive brake fluids are defined by the Federal Motor Vehicle Safety Standards and are assigned Department of Transportation DOT numbers Qualities that brake fluid must have e Free flowing at low and high temperatures A boiling point over 400 degrees F 204 degrees C Low freezing point Non corrosive to metal or rubber brake parts Ability to lubricate metal and rubber parts Hygroscopic Ability to absorb moistur
19. ir pressure to pass through the air inlet valve to the diaphragm backside The combination of atmospheric pressure on the backside and vacuum on the front side then moves the diaphragm and master cylinder pushrod forward to apply the brakes fig 22 Hydraulic Assist Boost System Hydraulic brake assist systems are used on many vehicles with limited underhood space or vehicles with engines that cannot consistently produce sufficient vacuum to operate a vacuum power boost system These include Diesel engines Turbocharged engines Engines that operate at high load low vacuum such as truck applications Introduction to Brake Systems Study Guide Vacuum port closed One way check valve Master cylinder pushrod gt Air enters here Air inlet valve open Vacuum Brake Booster Applied fig 22 Vacuum Port Closed adei Return line Steering pump mes High pressure and reservior Hydro boost booster Steering gear and master cylinder fig 23 Hydraulic Assist System The most common type of hydraulic assist system uses pressure from the power steering pump to provide power brake assist Power steering pump pressure is used to both help apply the brakes and also to charge an accumulator for engine off assist fig 23 When hydraulic pressure fills an accumulator it pushes a rubber seal against a piston and collapses the internal spring If the power steering pump stops the engine quits the
20. ite the release handle pulled if available See symptom chart in the appropriate service manual Make a detailed visual inspection Make all necessary repairs BELONG Proceeding Problem is Problem is likely in front likely in rear brakes brakes e See symptom tables in the appropriate service manuals e Make all necessary repairs Portions of materials contained herein have been reprinted with permission of General Motors Corporation Service Operations License Agreement 0410610 29
21. le is rotated the thimble and spindle move in or out on precision threads fig 32 Reading an English Micrometer A One Inch micrometer always measures a fraction of one 1 inch and measurements are made in four easy steps Step 1 Read the largest number that is exposed on the sleeve as shown at right In this case the largest number exposed is 2 Record this value in step 1 as 0 2 inch 200 1000 inch Step 2 Count the number of marks that are exposed to the right of the number 2 as shown Each mark equals 0 025 inch In this example one mark is exposed for a total of 025 inch 1 X 0 025 0 025 Record this in step 2 as 0 025 inch 25 1000 inch Step 3 Next we find the line number on the thimble that aligns with or is just below the horizontal line on the sleeve The sleeve line in this example is between 15 and 16 Record this in step 3 as 0 015 inch 15 1000 inch Step 4 The Vernier Scale for the most precise measurement is located on top of the sleeve Look for the one Vernier line that most perfectly aligns with any line on the thimble In this case Vernier line number 3 aligns with the thimble most closely Record this in step 4 as 0 0003 inch 3 10 000 inch _14 Introduction to Brake Systems Study Guide Locknut Sleeve Thimble Ratchet Spindle f d i m Ee see 0 0 1 inch micrometer fig 32 Major Parts of an English Micrometer 2004 Melior Inc Introductio
22. level in either chamber could be either the result of normal brake lining wear or it could indicate an external leak External leaks visually check the master cylinder fig 60 calipers fig 61 and or wheel cylinders fig 62 for leaks Fluid will be visible on the face of the power brake booster if the master cylinder is leaking from its rear seal fig 61 The bottom of the caliper will be Introduction to Brake Systems Study Guide damp with fluid if a disc brake is leaking _ 26 fig 62 The bottom of the backing plate will be damp with fluid if a wheel cylinder is leaking 2004 Melior Inc Introduction to Brake Systems Study Guide Note Leaking calipers or wheel cylinders can coat the brake pads or shoes with brake fluid and cause complaints such as wheel lock up brake squeal pulling and ineffective braking WARNING Leaks should be repaired before driving any vehicle e Parking brake operation With the key on and the engine off engage the parking brake The brake should apply with approximately half travel of the lever or pedal and the brake warning light should turn on Deactivate the parking brake and it should release immediately The lever or pedal should return to the release position and the warning light should turn off Any problems with the engagement or release of the parking brake could indicate a binding linkage e Brake warning light With the key on and engine off parking brak
23. lines connect to the master cylinder 2004 Melior Inc Introduction to Brake Systems Study Guide e Maintain a small amount of residual pressure in the brake lines and wheel cylinders e That pressure is then used to hold the wheel cylinder cups tightly against the cylinder and prevent air from being pulled past the cylinder cups as the brakes are released e Not used on disc brakes as they would cause the disc brake pads to drag upon release Master Cylinder Leaks Master cylinders are subject to two types of leaks External and Internal e External leaks brake fluid can usually be seen running down the face of the power booster e Internal leaks the brake pedal will usually fall away when foot pressure is applied Refer to specific vehicle information for leak diagnostics and servicing procedures Balance Control Systems Many late model vehicles are equipped with front disc brakes and rear drum brakes and are generally heavier in the front than in the rear As a result different pressures are sometimes required between the front and rear to ensure even braking These are some of the items to be aware of concerning this braking differential e Disc brakes can apply at lower pressures that drum brakes e Metering valves are used to prevent the front disc brakes from applying before the rear drum brakes e lf the same hydraulic pressure is simultaneously applied to both the front disc and rear drum brakes during heavy brake
24. ms Study Guide e Move the anvil leg of the drum micrometer along the graduated shaft until the whole number of the drum diameter in this example 11 is aligned on the shaft Tighten the lock screw e Next move the dial indicator leg along the graduated shaft until the whole number of the drum diameter 11 is aligned on that side e Now move the dial indicator three 3 additional notches outward you will feel a click at each notch and tighten its lock screw Note Each notch is equal to precisely 1 8 0 125 inch and also aligns with a mark on the shaft Therefore 3 notches are equal to 3 x 0 125 inch 0 375 inch The drum micrometer is now set to the new drum diameter of 11 375 inches In actual use it may be necessary to move the dial indicator leg one notch in either direction from this point since all new drums don t come in increments of 125 inch e Touse the micrometer place it inside the drum and hold it spy ne ht flat against the rim of the drum fig 45 e Hold the anvil end steady against the inside of the drum and move the dial or right end back and forth slowly to obtain the highest reading e The highest reading achieved is the amount that the drum is oversized given in thousandths of an inch DIAL SHOWS a S OVERSIZE OR ae OUT OF ROUND fig 45 Measuring a Brake Drum Diameter In this case we add the dial indicator reading 0 015 inch to the original drum diameter setting of our m
25. n to Brake Systems Study Guide Now we simply total the numbers recorded in step 1 through step 4 Step 1 0 2 Step 2 0 025 Step 3 0 015 Step 4 0 0003 Total 0 2403 inch 2403 10 000 inch Note This example was performed with a one inch micrometer for demonstration purposes If the micrometer being used on a crankshaft journal for instance were a 4 inch tool then the measurements would be performed exactly as shown above except that three inches would be added to the measurement total Likewise a 2 inch micrometer would add one inch a 3 inch micrometer would add 2 inches etc Reading Metric Micrometers Reading a metric micrometer is similar to reading an English micrometer except that there are typically only three readings instead of four e On a metric micrometer the upper scale of the sleeve measures in 1 0 mm increments while the lower scale measures 0 5 mm increments e The thimble is divided into fifty 50 equal parts of 0 1 mm each which means that one 1 complete revolution of the thimble equals 0 5 mm e Note that metric micrometers do not have a Vernier scale e Inthe example shown here the upper scale reads 5 0 mm the lower scale reads 0 5 mm and the thimble reads 0 28 mm for a total of 5 78 mm 5 0 0 5 0 28 5 78 mm Recorded Totals Step 1 5 00 mm Step 2 0 50 mm slep 3 0 28 mm Total 5 78 mm Step 3 Drum Brakes A drum brake unit consists of two brake shoes mounted on a s
26. ned inside the rotor hat section Brake shoe e The rear caliper does not have to Assembly Rotor hat perform both service and parking brake celal functions e The parking brake shoes for this system are manually adjusted when installed a and theoretically should never need fig 55 Drum In Hat Parking Brake readjustment Note Kinked or binding parking brake cables or linkages could prevent a parking brake from applying or releasing properly This could also cause the brake warning light to remain on even after the parking brake is released Brake Lines and Hoses e Transfer fluid pressure from the master cylinder to the brake calipers and wheel cylinders e Onsome vehicles pressure can exceed 1000 psi e Brake hoses the shorter rubber covered sections that connect at the wheels must also maintain a high degree of flexibility e Because of the pressures involved only double walled steel brake tubing is approved for use in brake lines Warning Never use copper tubing as a replacement It cannot withstand the high pressure or the vibration to which brake lines are exposed Fluid leakage and system failure can result When replacing a brake line it is advisable to purchase a preformed OEM replacement as they are of the correct length bend and strength to handle the system demands It can also be more cost effective since bending and flaring brake lines can be quite time consuming In addition to OEM replacements aftermarket line
27. nes And Hoses Split Hydraulic Systems A partial loss of brake pressure makes it difficult or even impossible to apply the brakes Therefore federal law requires that all vehicles have two separate and independent hydraulic systems In this way the failure of one system will not result in a complete brake loss even though braking will still be severely reduced The two split systems used almost exclusively are e Diagonally split used on most front wheel drive vehicles e Front rear split used on most rear wheel drive vehicles On a diagonally split system fig 3 the left front and right rear brakes LF RR are connected to one channel of the master cylinder while the right front and left rear brakes RF LR are connected to the other channel of the master cylinder This system is typically installed on front wheel drive vehicles because they have a front heavy weight distribution and approximately 70 of the braking occurs at the front brakes As such if one part of a diagonal system failed the overall braking would only be reduced to 50 rather than to 30 if both front brakes were lost Diagonally split systems also use proportioning valves either in the master cylinder circuits or in the rear brake lines to maintain the proper front to rear pressure balance Proportioning valves will be covered in a later section On a front rear split system fig 4 both front wheel brakes work together on one system channel while both rear whe
28. ns and the fluid level in the master cylinder must be checked often e Technician 1 begins the procedure by pressing the brake pedal to build up fluid pressure e Technician 1 then continues to hold a steady pressure on the brake pedal while Technician 2 opens the bleeder valve and observes as fluid and air bubbles are expelled The brake pedal will go to the floor and Technician 1 will continue to hold steady pressure on the brake pedal until Technician 2 closes the bleeder valve e This process is repeated until all the air has been expelled Repeat the process for all four 4 wheels e Check and refill the master cylinder between wheels and when the procedure is completed Test the brakes to make sure the pedal is firm before driving the vehicle Note Do not allow the master cylinder to run out of fluid or air will be introduced into the system Note A valve depressor tool may be required to bleed the front disc brake calipers on vehicles equipped with a metering valve Bleeding Order One of the oldest adages in the automotive service industry involves brake bleeding The old saying is that you should bleed the brakes starting farthest from the master cylinder and move progressively closer with each wheel However few understand where this belief originated As with most firmly held beliefs there is some truth in this one but it is no longer universally applicable as it once was The history is this before front wheel drive ve
29. nstalled on a vehicle The maximum refinish diameter specification lets the technician know that there is enough material remaining on the drum to be used safely and without an increase in the potential for failure Maximum refinish diameters vary between drums The actual specifications are available in the applicable service manual for the vehicle being repaired e Discard diameter Maximum discard is the diameter to which a drum can be reused f not refinished f a drum exceeds the maximum discard diameter either from refinishing or through normal wear it must be discarded The maximum discard specification is usually stamped or cast into the drum surface fig 43 The difference between the maximum refinish specification and the maximum discard specification is the amount that must be allowed for the drum to wear after refinishing e Adrum micrometer as shown in figure 44 is required to accurately measure a brake drum s diameter Discard diameter Lock Screw Lock Screw Discard diameter value could be either metric or inches fig 43 Maximum Discard Specification fig 44 English Drum Micrometer Dial Procedure for reading an English drum micrometer Refer to figures 44 and 45 and use the measurement specifications below for the example Original new drum diameter 11 375 inches Maximum refinish diameter 11 435 inches Discard diameter 11 465 inches 18 2004 Melior Inc Introduction to Brake Syste
30. nt Rotation of the nut also takes up clearance as the brake pads wear Ball and Ramp Parking Brake In the ball and ramp park brake system fig 54 the caliper lever is attached to a shaft inside the caliper that has a small plate on the other end A second plate is attached to a thrust screw inside the caliper piston Three 3 steel balls separate the two 2 plates e When the parking brake is applied the caliper lever rotates the shaft and plate e Ramps on the surface of the plate force the balls outward against similar ramps in the other plate e This action forces the thrust screw and piston outward applying the brake e When the park brake is released an adjuster nut inside the piston rotates on the thrust screw to take up excessive clearance and provide self adjustment 23 2004 Melior Inc Introduction to Brake Systems Study Guide Piston seal Parking Brake Mechanism i Operating Lever Automatic Piston P x gums i Parking brake nboar level Outboard brake pad A ey brake pad Rotor iston Actuator Cone Nut screw fig 53 Screw and Nut Parking Brake Mechanism Drum in Hat Parking Brake Some later model vehicles with four 4 wheel arani disc brakes use a small drum brake incorporated fyo S into the rear rotor fig 55 called a drum in hat Q Caliper system ae e Consists of a simple cable activated wae shoe assembly that applies against a _ Rotor drum machi
31. o build pressure in the primary chamber and lines As the brake pedal is depressed further the pressure continues to increase e Fluid pressure between the primary and secondary piston then forces the secondary piston forward and pressurizes the fluid in the secondary circuit e lf the brakes are operating properly the pressure will be the same in both circuits Secondary Secondary Primary 5 Reservoir AAA N l Reservoir Reservoir san ll a QUU BZ Wisan NN E i Primary Reservoir 7 To Brakes fig 7 Brakes released left Brakes applied right If there is a leak in one of the brake circuits that Seontinw Primary circuit will not be able to maintain pressure Reservoir Figure 8 shows what happens when one of the circuits develops a leak In this example the leak is in the primary circuit and the pressure between the primary and secondary pistons is lost This pressure loss causes the primary piston to mechanically contact the secondary piston and the master cylinder now behaves as if it has only one piston The secondary circuit will continue to function correctly however the driver will have to press the pedal further to activate it In addition since only two wheels now have pressure the braking power will be reduced fig 8 Master Cylinder with a Primary Circuit Leak Reservoir Compensating Ports e Small holes that are located between the master cylinder rese
32. r during braking Typical Rotor Tolerances Lateral runout 0 0 003 in 0 08 mm Thickness variation O 0 0001 in 0 00254 mm Refer to vehicle service manual for specifications Measuring Rotor Lateral Runout Excessive heat can cause rotors to warp This warpage or lateral runout can cause braking problems and must be measured to determine if turning or cutting the rotors is required With the rotor mounted on the vehicle a dial indicator is used to determine the runout The amount the dial indicator needle deflects while the rotor is rotated is the lateral runout Suppose that in figure 28 the dial indicator needle moved left 0 002 inch 0 051 mm and then right 0 003 inch 0 076 mm The total runout is therefore 0 005 inch 0 127 mm and fig 28 Lateral Runout Measurement could indicate that the rotor needs to be refinished or replaced refer to vehicle service manual for specifications _12 2004 Melior Inc Introduction to Brake Systems Study Guide Measuring Rotor Thickness Variation Rotors must be measured for thickness variation if the customer is experiencing a problem with a vibrating or pulsating brake pedal Thickness variations can be caused by excessive heating and cooling of the rotor and even a small variation can cause an adverse braking condition Precision brake micrometers must be used when measuring the thickness variation of brake rotors A difference of more than 0 0003 inc
33. replacing brake hoses Defective or damaged hoses can balloon or swell which will store brake fluid pressure and cause the vehicle to pull during braking or give a low pedal concern A blocked restricted or kinked brake hose can also cause the vehicle to pull during braking In this case the pull will be to the Kose Nari opposite direction of the eaten flex hose problem component That means that a left front hose that is blocked restricted or kinked would cause a pull to the right and a right front hose failure would cause a pull to the left during braking Restricted A flapping flex hose kinked i flex hose d fig 58 Brake Hoses and Failure Modes 25 2004 Melior Inc BASE BRAKE DIAGNOSIS Diagnosing brake problems can be simplified by following a few basic steps Listen to the customer Verify the complaint Perform a visual inspection Conduct a brake pedal check Test Drive before and after repairs Listen to the customer ask the necessary questions Reservoir lid What kind of problem did the customer experience Under what conditions did the problem occur Does the problem occur with every brake apply Visual inspection Brake Fluid Fluid levels remove the master cylinder reservoir cover and inspect the fluid level in both chambers fig 59 Some reservoirs have both Hot and Cold fill level indications Reservoir make sure you use the correct level Low fluid
34. rvoir and the front side or pressure side of the master cylinder pistons e When the master cylinder pistons are in the at rest position no braking figure 9 the piston seals uncover the compensating ports and open the passages between the reservoir and the wheel brake channel e Allow for the normal expansion and contraction of brake fluid due to changes in temperature e Assist in fluid return after brake release See Bypass Port section below Note When the brakes are released the piston seals on both the primary and secondary pistons are located between the compensating port and the bypass port During braking the piston seals close the compensating port passages to the reservoir which prevents high pressure fluid from entering the reservoir 2004 Melior Inc Introduction to Brake Systems Study Guide Bypass Ports e The bypass ports like the compensating ports are passages that are open between the reservoir and the master cylinder chambers fig 10 However the bypass ports are open to the low pressure or back side of the pistons e Allow the master cylinder pistons to return to the at rest position rapidly Low Pressure Areas Bypass Port RAYATANAN AT AL ALE pA nAaaaaaaa raAgaRaaaAGaA AVAL ye B D Soooosodg WN elelelelelelee B yn FETA AAV VUUVLVLULUG A Bypass Port Bypass Port Pi aiim 7 Primary Piston Primary Piston N Secondary Piston W Piston Seal Secondary Piston
35. s are also available in various lengths and diameters Many are also pre flared and have flare nuts installed fig 56 Aftermarket bulk brake line is also available which can be cut to length and flared as needed Keep in mind that this is a job to be performed only by those with the requisite tools 24 2004 Melior Inc Introduction to Brake Systems Study Guide and skills Additionally care must always be exercised when bending any brake line so as to not kink and weaken the line All vehicles utilize one of two methods of brake line connection flaring fig 57 and each requires its own special tools and flare nuts They are e ISO International Standards Organization flare e Double flare ISO style flare Double inverted style flare ISO metric XN Inverted double flare Armored fig 56 Brake Tubing with Flared Ends and Flare Nuts Installed fig 57 ISO and Inverted Double Flares Brake Hoses e Designed to distribute high pressure brake fluid to the wheel brakes fig 58 e Must also allow for the vertical movement of the suspension and the side to side motion of the front wheels as the driver steers the vehicle e These forces are substantial and can weaken the hoses over time When performing brake service always remember to e Inspect the hoses for damage kinks or ballooning e Inspect hoses for proper routing e Never hang a caliper from a rubber brake hose e Replace the copper sealing washers when
36. s to have a duo servo brake system Self adjusters Drum brake systems have a self adjust Capability to compensate for wear on the brake shoes e Self adjuster mechanism consists of a series of links springs retainers and a star wheel screw adjuster e Rotational action of the brake shoes activates the self adjuster linkage when the brakes are applied and the vehicle is moving in reverse fig 41 fig 41 Brake Adjuster Operation duo servo Leading trailing Brake Operation Wheel Cylinder Three major differences between duo servo Star Adjuster Wheel and leading trailing drum brakes 1 Leading trailing systems have the anchor pin mounted at the bottom of the backing plated rather than at the top fig 42 2 Neither shoe pushes against the other in leading trailing 3 Leading trailing drum brakes are automatically adjusted when the parking brake is applied and released Trailing Shoe Leading Shoe fig 42 Typical Leading trailing Brake The operation of leading trailing brakes is much simpler than duo servo systems e When the brake pedal is pressed a wheel cylinder pushes equally on each brake shoe e n turn this forces the top of each shoe outward toward the drum and each shoe pivots on the anchor located at the bottom of the backing plate e Drum friction pulls the leading forward shoe into tighter contact with the drum and aids the hydraulic force of the wheel cylinder This action provides mos
37. stalling reinstalling it on the vehicle One method for bench bleeding a master cylinder requires attaching two 2 brake lines to the master cylinder fig 47 and directing them back into the reservoir Fill the reservoir s with clean DOT brake fluid and slowly push the master cylinder pistons in several times until air bubbles are no longer seen This procedure will save time and fluid when bleeding the hydraulic system after the master cylinder has been reinstalled Wa A 2 G 1 e fig 47 Bench Bleeding Master Cylinder 20 2004 Melior Inc Introduction to Brake Systems Study Guide The two 2 most commonly used methods for bleeding brakes are Pressure Bleeding A pressure bleeder is a special tank that is divided into two 2 chambers by a rubber diaphragm The upper chamber is filled with ee clean fresh DOT 3 brake fluid while the lower E chamber is pressurized with air The rubber diaphragm keeps the brake fluid separated from the air fig 48 Pressure bleeding is often the preferred method for bleeding brakes since one person can do the job alone and the master cylinder does not have to be repeatedly refilled during the process Pressure bleeding Manual bleeding Rubber yy Diaphragm fig 48 Typical Brake Pressure Bleeder Pressure bleeders are attached to the master cylinder with a hose and a special adapter fig 49 The special adapter seals the pressure bleeder to
38. t of the braking force e The secondary shoe is not self energizing as in Duo servo but does provide some braking force due to the action of the wheel cylinder When backing up the opposite action takes place In leading trailing systems both the primary and secondary brake linings are typically identical in size As a general rule of thumb the lighter a drum brake equipped vehicle is the more likely it is to have a leading trailing brake system 7 2004 Melior Inc Introduction to Brake Systems Study Guide Brake Drums Any time brake service is performed all brake drums should be inspected for the following e Excessive wear or scoring Hot spots or heat checks Out of round Distortion Cracks Any brake drum that is cracked must be replaced Those that have hot spots distortion or are out of round can cause braking problem such as pulling vibration chatter noise and pulsation Many times these drums can be refinished and reused other times they must be replaced Drums that exhibit minor scoring but have no other problems can sometimes be reused without refinishing however it is critical that the diameter of a drum be measured to determine if it can be safely refinished and reused Measuring Brake Drums When measuring brake drums to determine if they can be reused there are two specifications that must be understood e Maximum refinish diameter the maximum diameter to which a drum can be turned and still be rei
39. t really doesn t matter which but most people start with the right rear then move to the other rear brake That will completely bleed that system the rears Then bleed the front brakes Again the order is really unimportant If the vehicle has a diagonally split system you can begin wherever you like but and this is important whichever you do first the opposite brake must be next to prevent the possibility of aeration If the right rear is first the left front must be next Start with the right front Then the left rear is second Most seasoned technicians will typically go from right rear to left front then from left rear to right front That is usually a good idea since a consistent procedure helps to avoid errors Note because of the nature of some Antilock Brake systems certain vehicles will use a specific bleeding order to help ensure that all of the air is removed from that unit If that is the case follow the manufacturer s instructions So 2004 Melior Inc Introduction to Brake Systems Study Guide Parking Brake Federal Motor Vehicle Safety Standards require that automotive parking brakes be capable of holding a vehicle stationary on a 30 degree grade The parking brake systems on most vehicles use either a hand or foot operated lever and cables to mechanically apply the rear wheel brakes fig 51 Parking Brake Drum Duo Servo When a driver applies the parking brake on a vehicle equipped with rear drum brakes
40. tationary backing plate When the brake pedal is pressed a hydraulically activated wheel cylinder pushes the shoes out to contact a rotating drum which creates friction and slows the vehicle As the pedal is released return springs retract the shoes to their original position e Late model vehicles that use drum brakes will have them only on the rear wheels e The two most common types of drum brakes that we will deal with are duo servo and leading trailing _15 2004 Melior Inc Duo servo Brakes Consist of the following components fig 38 Brake drum One primary shoe and one secondary shoe with friction linings Hydraulic wheel cylinder Anchor pin Backing plate Adjusting screw Return springs hold down springs connecting springs Adjusting linkages and springs Wheel Cylinders Consist of the following parts fig 39 Cylinder Two pistons Two lip seal piston cups Expander spring assembly Two protective dust covers Two actuating pins some models Bleeder valve Duo servo Brake Operation Introduction to Brake Systems Study Guide Wheel Cylind Retum Backing Plate Spring qa aian OTT i EPM gt i fig 38 Duo servo Drum Brake Actuating Pin if equipped Cylinder Boot Body Piston Bleeder A Valve ae Spring Assembly Se awe i Expander y Seal Piston Actuating Pin if equipped Boot fig 39 Wheel Cylinder When duo servo brakes are in the rel
41. the master cylinder to prevent fluid and air leaking into or out of the system To pressure bleed a brake system Fill the pressure bleeder with clean DOT 3 brake fluid and charge the air reservoir with fifteen 15 to twenty 20 psi 105 140 kPa of air Fill the master cylinder reservoir to the proper level usually about one quarter 1 4 inch from the top Install the bleeder adapter to the master cylinder reservoir attach the supply hose from the pressure bleeder to the adapter and open the supply valve Attach a bleeder hose to the wheel cylinder bleeder valve and extend the end of the hose into a clear glass container partially filled with DOT 3 brake fluid fig 50 Open the bleeder valve Any air trapped in the system can be seen as bubbles escaping from the bleeder hose Close the bleeder valve after all air bubbles have been expelled Repeat the bleeding process for all four 4 wheels Remove the pressure bleeding equipment and fill the master cylinder to the proper level Test the brakes to make sure the pedal is firm before driving the vehicle Note A valve depressor tool may be required to bleed the front disc brake calipers on vehicles equipped with a metering valve fig 49 Pressure Bleeder w Master Cylinder Adapter fig 50 Air Bubbles Bleeding from a Brake System Da 2004 Melior Inc Introduction to Brake Systems Study Guide Manual Bleeding The manual brake bleeding process requires two 2 technicia
42. ting Vacuum Check tee Fi aaa Jia Single Diaphragm Booster Tandem Diaphragm Booster fig 19 fig 20 For simplicity we will refer to a single diaphragm vacuum brake booster in this example Vacuum boosters are mounted Vacuum port open between the brake pedal R pushrod and the master cylinder i L N and receive engine vacuum One way check 5 gt Atmospheric pressure through a hose and check valve gt valve one way valve The check valve holds vacuum BreA pressure and assures power eira assist capability during times of low engine vacuum i e the engine quits With the check valve in place a booster will have enough reserve vacuum for 2 3 brake applications after engine vacuum is lost fig 21 Diaphragm return spring Air inlet valve closed Vacuum Brake Booster fig 21 Booster at Rest with Vacuum on Both Sides of Diaphragm Vacuum boosters operate as follows e When the brake pedal is released an internal vacuum port is open which allows engine vacuum to flow from the check valve to both sides of the diaphragm With equal pressure vacuum on both sides the diaphragm is held to the rear by spring pressure fig 21 2004 Melior Inc As the brakes are applied the brake pedal pushrod moves forward which closes the vacuum port and opens the air inlet valve This action seals off the backside of the diaphragm from the vacuum source and at the same time allows filtered atmospheric a
43. unt of brake force applied they also determine the travel distance of the different pistons For instance if the one square inch master cylinder piston moves one inch a one square inch wheel cylinder piston will also move one inch with the same force fig 5 If that same one square inch master cylinder piston moves one inch then a two square inch wheel cylinder piston twice the size will move just one half inch half the distance but with twice the force fig 6 1 INCH TRAVEL INCH TRAVEL a 1 2 INCH TRAVEL fig 5 Piston Area and Travel 1 inch fig 6 Piston Area and Travel 1 2inch Master Cylinder e Converts mechanical force from the brake pedal power booster and push rod into hydraulic pressure e Contain pistons piston seals return springs and internal brake fluid ports e Also has a fluid reservoir that may either be an integral part of the unit or remotely mounted The reservoir itself will have a removable cap with a rubber diaphragm seal that must be in good condition to seal properly e Most reservoirs also have a low brake fluid level switch to alert the driver of a low fluid condition 2004 Melior Inc Introduction to Brake Systems Study Guide The Master Cylinder in Action As you can see in figure 7 there are two pistons primary and secondary and two springs inside the master cylinder e When the brake pedal is pressed a push rod moves the primary piston forward which begins t
44. ventually becomes high enough to overcome the larger outlet piston and the valve opens again allowing additional pressure to the rear brakes The reopening of the valve then increases pressure on the outlet side which again closes the valve This cycle is repeated several times a second and keeps the pressure to the rear brakes proportionately less than the pressure to the front disc brakes The proportional cycling action therefore makes for more positive braking under adverse conditions 2004 Melior Inc From master cylinder Normal Braking fig 15 Proportioning Valve Open During Normal Braking Pressure Differential Valve Switch e fig 17 Spring loaded plunger valves that are used to turn on dashboard warning lights if hydraulic pressure is lost in either channel of a hydraulic brake system e Once the pressure loss problem is repaired some valves will automatically reset themselves while others must be bled to reset Refer to the appropriate service material for the applicable procedure From master Introduction to Brake Systems Study Guide cylinder Hard Braking fig 15 Proportioning Valve Modulating During Heavy Braking PRESSURE BALANCE High Pressure High Pressure PRESSURE IMBALANCE Hydraulic failure in front brake circuit Low or 0 Pressure High Pressure fig 17 Pressure Differential Valve The metering valve proportioning valve and the pressure differential switch
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