Class 5 to 7 Truck and Bus Hydraulic Brake System
Contents
1. TIP Possible Causes of Brake Drag 1 Booster does not return 4 Master cylinder does not return 2 Brake pedal does not return 5 ABS traps pressure 3 Brake hoses and tubes collapsed or kinked 6 Brake caliper does not release Remove filler cap from the master cylinder Look at the diaphragm inside the cap Is the diaphragm swollen See Figure 4 With engine OFF measure location of the pedal from the floor Master cylinder has been contaminated Start engine and measure if pedal dropped to a new position with the wrong fluid Shut OFF engine and measure if pedal returned to original 7 Replace all of the following uae position cylinder all calipers ABS hydraulic unit and all rubber hoses Flush the steel brake tubes with clean brake fluid prior to installing new brake Did pedal drop about 1 2 components when engine started No Two people will be needed for this step Apply and release brakes then quickly loosen bleed screw at the ee Aine suspected caliper s probably with the The bleed screw must be loosened quickly in case trapped pressure power steering pump at the caliper bleeds down before the bleeder screw is loosened Does fluid spurt out under pressure Tighten bleed screw to manufacturer s recommended torque The problem is probably with the booster master Remove caliper and shoe pads cylinder or ABS For rear axle disconnect axle from hub Does rotor spin freely Continued on page2. work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 7 BOSCH Booster Backup Pump Does Not Run TIP The booster backup pump will not run if there is no voltage to the motor or if the motor is damaged e g burned out or jammed A no voltage condition can occur because 1 Battery is dead 2 Relay is stuck 3 Wires are broken battery to motor circuit or ignition switch brake light switch to booster flow switch circuit 4 Booster flow switch is not closed to ground Is there proper voltage to the booster backup pump Disconnect booster flow switch connector 6B the problem is probably with Check continuity from relay to connector and the booster backup pump Check continuity through flow switch Is wire continuity OK No Yes 7 The problem is Is flow switch probably with the wire continuity OK Yes The problem is Is there voltage to probably with the the relay coil booster flow switch Check ignition relay circuit and brake light Is there battery switch to relay circuit for continuity and voltage to the relay properly functioning switches 8 The problem is probably with the relay The problem is probably with the battery to relay circuit Check it for continuity NOTE Use appropriate brake component or vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 8 BOSCH Brake Pedal Continues
3. 17 No Yes The problem is probably Caliper assembly is with bearing pre load probably dragging Continued on next page NOTE Use appropriate brake component or vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 15 BOSCH Brake Drag continued From previous page Replace caliper without shoe pads and tighten slide pin bolts in proper sequence and to correct torque Slide caliper in and out by hand Does caliper move freely 19 The problem is probably with the pins if Remove caliper and replace shoe pads pin slide or rails if rail slide type brake Slide shoe pads side to side Do shoe pads move freely 20 21 The problem is probably The problem is probably with with the shoe pads the pistons in the caliper NOTE Use appropriate brake component or vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 16 BOSCH Brake Drag continued TIP Prior to moving master cylinder 1 8 inch away from booster loosen two diagonally opposed nuts that hold the master cylinder to the booster by 1 8 inch Have a power tool ready to loosen the remaining two nuts after the brakes have been applied and released From page 15 Two people will be needed to do this step Apply and release brakes then quickly loosen primary brake line tube nut at the master
4. Class 5 to 7 Truck and Bus Hydraulic Brake System Diagnostic Guide 1st Edition BOSCH Important Service Notes The information in this publication was current at the time of printing The information presented in this publication is subject to change without notice or liability The information contained in this publication is intended for use by properly trained and equipped professional technicians It is NOT for the Do It Yourselfer Failure to follow safety and repair procedures can result in personal injury or damage to vehicles components and equipment AWARNING Failure to follow all the safety and vehicle repair procedures either con tained in this manual in the chassis and vehicle manufacturer s repair manuals or in accordance with other accepted methods can result in personal injury death or damage to components vehicles or personal property Correspondence concerning this manual should be addressed to Robert Bosch Corporation Attn Hydraulic Actuation amp Truck Brake Engineering 401 North Bendix Drive South Bend Indiana 46628 FAX 574 237 2210 August 2003 BOSCH Contents TTF Baraat yu EE OA es Wee een GH STE ONG n CeO a Ver Oe 3 Purpose of this diagnostic guide Using this diagnostic guide Exclusions Brake ey SIM TOOT orisi aa EE ERE E EE EE EE EE EE EE 4 Schematic of Typical Electrical SySteM ssesissesirsisiisseeirsirissrsisrrrsririsriisrissiririssrrssiis 5 Brake Warning Light an
5. curately and quickly diagnose the most likely causes of a customer s brake related complaint This diagostic guide is NOT a repair instruction but only a guide When the probable cause of a customer s complaint is established the repair procedure must be done in accordance with the instructions in the vehicle manufacturer s service manual Using This Diagnostic Guide The brake technician must be able to understand the customer s complaints and description of the symptoms well enough to match them to one or more of the categories listed in the Table of Contents Most customer complaints and symptoms on the vehicle can be matched to these categories Each category has a corresponding flow chart that will lead the brake technician to the most likely cause of the customer s complaints and vehicle symptoms Several of the flow chart paths end with a comment box that also includes a number Brake technicians can use this number to document why a Bosch manufactued part was replaced IMPORTANT REMINDER The first step in diagnosing any customer complaint is to confirm the customer s complaint and determine which category applies The flow charts show which diagnostic steps need to be taken The brake technician must have the necessary skills needed to perform each step The flow charts are structured to guide the brake technician to take the quickest and easiest steps first Often these first simple steps will be enough to determine what repair need
6. cylinder The tube nut must be loosened quickly in case trapped pressure at the master cylinder bleeds down before the tube nut is loosened Does fluid spurt out under pressure at the brake line tube nut Two people will be needed for this step Tighten tube nuts to manufacturer s recommended torque Apply and release brakes Shut off engine After releasing brakes quickly loosen the four nuts see TIP holding the master cylinder to booster by 1 8 inch Quickly pull master cylinder away from booster and quickly loosen primary brake line tube nut All of these steps must be done quickly in case trapped pressure at the master cylinder bleeds down before the tube nut is loosened Tighten brake line tube nut to manufacturer s recommended torque Apply and release brakes then quickly loosen secondary brake line tube nut Retighten tube nut The problem is Does fluid spurt out probably with under pressure at the the ABS brake line tube nut Continued on next page NOTE Use appropriate brake component or vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 17 BOSCH Brake Drag continued From previous page Does fluid spurt out under pressure at the tube nut The problem is probably with 22 The problem is probably the booster or pedal linkage with the master cylinder Retighten tube nut Retighten 4
7. d Buzzer Do Not SHUG vcccssscwsisnarconrsrrirensaarreouasvcnsssraagaocinacucnsaaniieiens 6 Booster Backup Pump Runs I OMUMUOUSIY wu scgsracnsnanncnevee suvisiaeroseonsosioanaynssaunesisnnavessounedavaneaiends 7 Booster Backup Pump Does Not RUNsnissncisrirosiiioreie aoa E ETES EE E 8 Brake Pedal Continues to Fall With Steady Foot POPC sccccsvcssessnaaseoryonrdensnanrvgeatenniasinnnadensionse 9 Brake Pedal Feels Spongy Soft OF SDAUIBV co ssssarsinsavinvessssnnepeniaoninvinnrnnnneyeinteniaennspradnontentneunioas 9 Brake Pedal Feels Very TAI esters sinncinilasepousecenasins serv naressesvasnsda tedium nie nsosle gave ean a 10 Potential Master Cylinder and Booster Leak POTIS csccosswcsmaicrneenausnveerndonctantionaawoasnnnc asinine lI EP ecstatic tpn neon Rev aan 12 Brake Drao raoa e canes eE e E AE EE E EE E E E E N 15 Master Cylinder Cap DIG iA OIH8 cn visssnnaossanessanncrsinasodian uesavasoasnancossoneysdansyhanwanosnansoananantsnnsielions 19 Low Shoe Pad Life or Uneven Shoe Pad Wear ocscvccossrrsersornnesrsarvsiaotarsnssouiorteswiannsapanasortnoinans 20 Printed in the United States of America Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide Copyright 2003 Robert Bosch Corporation 1 BOSCH Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 2 BOSCH Preface Purpose Of This Diagnostic Guide The purpose of this diagnostic guide is to assist Class 5 to 7 hydraulic brake repair technicians to more ac
8. eakage section master cylinder After repair differential l completion push Gini pressure switch very hard on the brake pedal to shut OFF differential pressure switch 1 The problem is probably with the master cylinder fluid level reed switch NOTE Use appropriate brake component or vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 6 BOSCH Booster Backup Pump Runs Continuously TIP The booster backup pump can run only if the relay is closed The relay will be closed only if 1 the flow switch is closed and there is power to the coil or 2 the relay is stuck closed Is ignition key ON while condition exists iti Unplug connector The problem is U om flow switch probably with the Is there voltage il brake Tight switch to the relay coil Is booster backup pump still running 3 The problem is probably The problem is Check for proper power 4 The problem is with the booster backup probably with the steering pump flow by probably with the pump relay ignition switch rapidly rotating the booster backup steering wheel The pump relay alternative is to measure the fluid flow Does steering wheel rotate normally Is flow OK The problem is The problem is probably with the probably with the power steering pump booster flow switch NOTE Use appropriate brake component or vehicle manufacturer s service manual for all repair
9. earing abnormally excessive inner vs outer shoe pad wear Are brakes dragging Do brakes show signs of heat For low brake operating Condition is probably temperature conditions related to the duty cycle see service manual for usage or the driver s appropriate alternate braking habits replacement shoe pads NOTE Use appropriate brake component or vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 20 Correspondence concerning this manual should be addressed to Robert Bosch Corporation Attn Hydraulic Actuation amp Truck Brake Engineering 401 North Bendix Drive South Bend Indiana 46628 Fax 574 237 2210 Printed in the United States of America Copyright 2003 Robert Bosch Corporation
10. ing to manufacturer s Does it still leak recommended torque Yes No a Replace tube seat or bleed screw Does it still leak Tighten to manufacturer s recommended torque Replace tube seat or fitting Does it still leak Tighten to manufacturer s recommended torque caliper No a aimee Does it still leak probably with the No Yes 11 The problem is probably with the caliper NOTE Use appropriate brake component or vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 12 BOSCH Leakage continued TIP Brake fluid mixes with water Power steering fluid floats on water From previous page Examine booster amp master cylinder assembly Is leak at booster to master cylinder interface See Figure 3 Determine if the leak is brake Is leakage at booster fluid or power steering fluid or master cylinder See TIP Booster Power Master Steering Fluid Cylinder Thoroughly clean the entire booster with soap amp water then dry it Start engine apply brakes with normal foot pedal force several times and look for a point of leakage Continue as necessary until leak is found Thoroughly clean the entire booster with soap amp water then dry it Start engine apply brakes Can the leakage be fixed by tightening with normal foot pedal force the tube nuts or replacing the tube seats several times a
11. kup pump run See Booster Backup Does the booster backup Pump Does Not Run pump sound like it slows section down when pushing harder on the pedal 10 The problem is The problem is probably with the probably with the ABS booster backup pump NOTE Use appropriate brake component or vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 10 BOSCH Potential Master Cylinder and Booster Leak Points Flow switch seal Booster outlet port Inlet port seal Reservoir to body interface Master cylinder outlet ports Booster housing to backup pump interface Figure 3 Potential Master Cylinder and Booster Leak Points NOTE Use appropriate brake component or vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 11 BOSCH Leakage TIP Most external leakage is easy to detect by wetness and or appearance of fluid drops However slight dampness no drops or wetness may not indicate a leak Is the leak at the booster master cylinder ABS hydraulic unit brake caliper or tubes and hoses Booster or Master Cylinder Caliper Continued on Is leak at the bleed screw See ABS or vehicle next page manufacturer s service manual No Tighten bleed screw to Is leak at the brake hose tube fitting manufacturer s recommended torque Tighten tube fitt
12. nd look for a point of leakage Continue as necessary until leak is found Continued on next page 13 pag Tighten tube nuts to Replace master manufacturer s cylinder recommended torque or replace tube seats as needed Continued on next page NOTE Use appropriate brake component or vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 13 BOSCH Leakage continued From previous page Is booster flow switch leaking No Yes 14 Is leak at booster inlet port The problem is probably with the flow switch O ring seal No Is leak at booster The problem is probably backup pump with the booster inlet fitting O ring seal Is leak at the interface between the booster housing and the backup pump Or if an adapter is present between the adapter and the housing or the adapter and the pump The problem is probably with the backup pump 16 The problem is Clean and examine all sealing surfaces and probably with the seal grooves at backup pump booster booster housing and adapter Are surfaces and grooves flat and free of damage No Yes T The problem is probably with 18h problem is probably the damaged component with the seals NOTE Use appropriate brake component or vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 14 BOSCH Brake Drag
13. nuts to 34 to 41 Nm Apply and release brakes then quickly do the following Remove pin that holds the pedal rod to the pedal arm Open primary tube nut at master cylinder These steps must be done quickly in case trapped pressure in the master cylinder bleeds down before the tube nut is loosened Does fluid spurt out under pressure at the tube nut 23 Retighten the tube nut Retighten the tube nut The problem is probably with The problem is probably the pedal linkage with the booster NOTE Use appropriate brake component or vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 18 BOSCH Master Cylinder Cap Diaphragms Diaphragm swollen due to exposure to mineral oil power steering fluid automatic transmission fluid motor il etc Normal diaphragm Figure 4 Comparison Swollen vs Normal Diaphragm NOTE Use appropriate brake component or vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 19 BOSCH Low Shoe Pad Life or Uneven Shoe Pad Wear TIP Shoe pad life can vary greatly due to many factors including vehicle type vocation e g rollback tow truck box truck bus etc terrain urban vs rural use and driver style Therefore no guidelines can be provided for the mileage that should be obtained Verify condition that shoe pads are w
14. s to be made AWARNING After completion of all diagnostic steps the brake technician must remember to tighten any tube nuts fittings bolts screws bleeder screws etc that were loosened as part of the diagnostic procedure In all cases the vehicle manufacturer s service manual must be used for any repair instructions 3 Brake system warning lights and buzzers are unique to the vehicle manufacturer The Brake Warning Light and Buzzer Do Not Shut Off flow chart offered in this guide is generic and may not apply to all vehicles The vehicle manufacturer s service manual must be consulted in order to determine the proper function of these warning devices Exclusions The ABS portion of the hydraulic brake system is not addressed in detail in this guide since the ABS hardware and software are unique to the specific vehicle manufacturer Any diagnosis or repair needs to be done in accordance with the vehicle manufacturer s ABS service manual Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 3 BOSCH Hydraulic Brake System Diagram Monitoring Device That Controls Warning Light and Buzzer Hydraulic Booster Ea Master Cylinder Brake Pedal and Linkage lt lt Au Power Steering Reservoir Power sisin ea Power Steering en Booster Backup Pump ABS Hydraulic Control Unit Figure 1 Brake System Components NOTE Use appropriate brake component or vehicle manufacturer s service manual for all repair
15. to Fall With Steady Foot Force Is there visible brake fluid leakage from master cylinder ABS hydraulic unit disc brake calipers watch for expanding piston boots tubes hoses or connections Is ABS light on i s ight on See Leakage section 9 The problem is probably due The problem is probably to internal bypass of brake with ABS hydraulic unit fluid inside master cylinder Brake Pedal Feels Spongy Soft or Springy TIP If the brake pedal does NOT fall with steady foot pressure but feels spongy soft or springy the problem is probably caused by air trapped in the brake fluid system Start by bleeding the brake system at the caliper furthest from the master cylinder and work from the back to the front of the vehicle NOTE Use appropriate brake component or vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 9 BOSCH Brake Pedal Feels Very Hard TIP The most common reasons for a very hard brake pedal are 1 Insufficient flow or pressure from the power steering pump 2 The ABS hydraulic unit is blocking the flow of brake fluid to the calipers Check for proper power steering pump flow and pressure If the steering wheel cannot be rotated rapidly or if flow is measured and found to be low the problem is probably with the power steering pump With engine OFF does brake pedal feel very hard Does the booster bac
16. work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 4 BOSCH Schematic of Typical Electrical System Flow switch hamess Fluid level ground optional indicator switch Flow switch Differential pressure switch Backup pump Ignition switch Chassis ground Relay Figure 2 Schematic of Typical Electrical System NOTE Use appropriate brake component or vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 5 BOSCH Brake Warning Light and Buzzer Do Not Shut Off TIP The light and buzzer come on together typically in response to signals from the parking brake switch booster flow switch master cylinder fluid level indicator switch the master cylinder differential pressure switch or booster backup pump See Figure 2 Is master cylinder reservoir more than 1 2 full No Yes Fill master cylinder reservoir with Is the master cylinder differential appropriate type pressure switch closed to ground brake fluid Are light and See Booster Backup Pump Is there a leak buzzer still ON Runs Continuously section In the brake or Booster Backup Pump fluid system Does Not Run section as appropriate Check brake fluid Check master Nos system for leakage cylinder fluid Continuity per Leakage section level reed switch Check shoe pads for for continuity wear The problem is Diagnose per probably with the L
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