Hydraulic_Brake_1241742054
Contents
1. 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 See Leakage section A Snap on master cylinder The problem is probably pressure test kit P N SVT 750 with ABS hydraulic unit may be used to determine if the master cylinder can build and hold pressure Does the master cylinder leak down The master cylinder outlet port sizes are 9 16 18 UNF 2B on the primary and 1 2 20 UNF 2B on the secondary in most cases An alternative to the test kit is to plug the master cylinder outlet ports Apply steady foot force to the brake pedal Does pedal continue to fall The problem is probably due to bypass of brake fluid inside master cylinder Brake Pedal Feels Spongy Soft or Springy 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 and vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 19 BOSCH Booster Backup Pump Does Not Run The booster backup pump will not run if there is no voltage to the motor or if the motor is damaged e g b2. and vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 10 BOSCH Over Heated Brakes continued From previous page Is problem at more than one corner Problem may be with Remove wheel if necessary Can outer actuation or ABS shoe be moved up and down by hand No Continued Problem may not be caliper Can inner shoe be moved up on Check wheel bearing per and down by hand page 12 manufacturers recommended o proceduure Yes Problem may be stuck caliper Replace caliper per manufacturer s recommended procedure Problem may be stuck pins Repair per manufacturer s recommended procedure NOTE Use appropriate brake component and vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 11 BOSCH Over Heated Brakes continued 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 an air ratchet ready to loosen the remaining two nuts quickly after the brakes have been applied and released From page 10 or 11 Two people will be needed to do this step e Apply and release brakes then quickly loosen primary brake line tube nut at the master cylinder e The tube nut must be loosened quickly in case trapped pressure at the master cylinder bleeds down bef
3. 2 Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 3 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 accurately and quickly diagnose the most likely causes of a customer s brake related complaint This diagnostic 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 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 firs
4. Class 5 to 7 Truck and Bus Hydraulic Brake System Diagnostic Guide 2nd Edition www bosch us BOSCH 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 and 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 Failure to follow safety and vehicle repair procedures either contained 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 Medium Heavy Truck Brake Engineering 401 North Bendix Drive South Bend Indiana 46628 FAX 574 237 2210 2nd Edition September 2006 Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 2 BOSCH Contents E E 6 a eR cn Oe A OOE A ane rr ee ey OA eee Purpose of This Diagnostic Guide ecient crear ete ee anna Using This Diagnostic Guide sessssesssseeesesssssssseccoereeesossssseeeerreessssssssecerrosssssssssssereonsssssssssereeess
5. EXCISION S eenn A E E A DiagraM Siirsin irois ereas rori tainas a EAEE ESEE EPESA E ARRESE EESE EVES ERE Ea Hydraulic Brake System Diagram s eessseesssssssssesssssseeeessseeeeesssssseossssseereessseereosssssseossssserressssereee Schematic of Typical Electrical System esesssssesssseeeeessssssssecoceeeeesssssssseereeeeeossssseeeerreeesssss Potential Master Cylinder and Booster Leak Points ss sssessesssseresssseeresssssssesssssseeeessssereee Potential COMplaiNntS ss sssssssessissisesessssssisstessissssssesrsssissrixsssssdsesusvosesnasoses st ssuesis eesesissi sin 8 Low Shoe Pad Life or Uneven Shoe Pad Wear sssssscsscsssscssscssessscesenscssessscaccssesscenserscsess Over Heated Brakes imen ere E E E E eee eee Leakage siadat creiceeens contrast E A tecereeneenin 15 Brake Pedal Feels Very Hard sss cssssssissvnssussanavssiibiastiactiasetevainsaiomsssnceneesodtonssnssisesinenuatbunnainions 18 Brake Pedal Feels Spongy Soft or Springy eessssssssssserresssssssccoereeesssssssseeereesssssssseseeese 19 Brake Pedal Continues to Fall with Steady Foot FOfC ssssssssssorseesssssssssseeeesssssssssesceese 19 Booster Backup PUMP Does Not Run cssssssssssssssscscssscsscssccescsscssccssenscsseeuccasesscseccnsesscense 20 Booster Backup PUMP RUNS Continuously essessessecssecsccsccsecssccsscnscssccsecstecsessccaeesseeses 21 Brake Warning Light and Buzzer Do Not Shut Off ssssssscssscssssscssecsscssecsessccssesseeees 2
6. hoe pads for wear Check master cylinder fluid level reed switch for continuity Continuity Does Not Run section as appropriate No Continuity The problem is probably with the master cylinder fluid level reed switch The problem is probably with the master cylinder differential pressure switch Diagnose per Leakage section After repair completion push very hard on the brake pedal to shut OFF differential pressure switch NOTE Use appropriate brake component and vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 22 Correspondence concerning this manual should be addressed to Robert Bosch Corporation ATTN Medium Heavy Truck Brake Engineering 401 North Bendix Drive South Bend Indiana 46628 Fax 574 237 2210 September 2006 Printed in the United States of America Copyright 2006 Robert Bosch Corporation
7. k of lining for lining type Uneven or high rate of wear Correct lining If less than 26K GVW in most cases should be HX 402 If 26K GVW or greater in most cases should be HX 7A1 Note These identifications may be found imprinted on the back of each shoe Apply correct lining See Over Heated per manufacturer s Brakes section recommendation NOTE Use appropriate brake component and vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 8 BOSCH Over Heated Brakes Typical over heated brake signs and symptoms may include brake lining odor steering wheel pull or blued rotors Uneven lining wear or rapid lining wear is often mistaken as stuck calipers More probable root causes are incorrect linings excessive brake duty or incorrect repairs when last serviced See page 8 Low Shoe Pad Life or Uneven Shoe Pad Wear Possible causes of over heated brakes 1 2 3 4 5 6 7 8 Booster does not return Brake pedal does not return Brake hoses and tubes collapsed or kinked Master cylinder does not return ABS traps pressure Brake caliper does not release There are two different guide pins that connect the anchor plate to the caliper Verify proper assem bly per manufacturer s recommended procedure Improper brake operation may be due to caliper interference with adjacent truck components as the caliper moves inboard as lini
8. ngs wear Verify that there is no interference NOTE Use appropriate brake component and vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 9 BOSCH Over Heated Brakes continued 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 Master cylinder has been contaminated with pedal from the floor the wrong fluid Start engine and measure if pedal dropped e Replace all of the following master cyl to a new position inder all calipers ABS hydraulic unit and Shut OFF engine and measure if pedal all rubber hoses returned to original position Flush the steel brake tubes with clean brake fluid prior to installing new brake components Did pedal drop about 1 2 when engine started Two people will be needed for this step The problem is probably with e Apply and release brakes then the power steering pump quickly loosen bleed screw at the suspected caliper s The bleed screw must be loosened quickly in case trapped pressure at the caliper bleeds down before the bleeder screw is loosened Does the fluid spurt out under pressure Tighten bleed screw to manufacturer s recommended torque Continued The problem is probably with Continued on the booster master cylinder on next page or ABS page 12 NOTE Use appropriate brake component
9. ore the tube nut is loosened Does fluid spurt out under pressure at the brake line tube nut Tighten brake line tube nut to Two people will be needed for this step manufacturer s recommended torque Tighten tube nuts to manufacturer s Apply and release brakes then recommended torque quickly loosen secondary brake line Apply and release brakes tube nut Shut off engine e Retighten tube nut After releasing brakes quickly loosen the four nuts holding the master cylinder to booster by 1 8 inch Does fluid spurt out under Quickly pull master cylinder away from pressure at the brake line tube 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 The problem is probably with the ABS bleeds down before the tube nut is loosened Continued on next page NOTE Use appropriate brake component and vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 12 BOSCH Over Heated Brakes continued From previous page Does fluid spurt out under pressure at the tube nut The problem is probably with the booster or pedal linkage with the master cylinder Retighten tube nuts Retighten 4 nuts to 34 to 41 Nm Apply and release brakes then quickly do the following e Remove pin that holds the pedal rod to the pedal arm e Open primary tube nut at master cylinder e These ste
10. ppropriate brake component and vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 5 BOSCH Schematic of Typical Electrical System FLUID LEVEL INDICATOR SWITCH FLOW SWITCH HARNESS d GROUND OPTIONAL FLOW SWITCH DIFFERENTIAL PRESSURE SWITCH BACKUP PUMP IGNITION SWITCH CHASSIS GROUND RELAY Figure 2 Schematic of Typical Electrical System NOTE Use appropriate brake component and vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 6 BOSCH Potential Master Cylinder and Booster Leak Points RESERVOIR TO BODY INTERFACE EmA mii FLOW SWITCH SEAL BOOSTER OUTLET PORT AE INTLET PORT SEAL E O CY i ff Wm ys QCLO 4 OP M MASTER CYLINDER OUTLET PORTS BOOSTER TO MASTER CYLINDER INTERFACE BOOSTER HOUSING TO BACKUP PUMP INTERFACE Figure 3 Potential Master Cylinder and Booster Leak Points NOTE Use appropriate brake component and vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 7 BOSCH Low Shoe Pad Life or Uneven Shoe Pad Wear Uneven lining wear or rapid lining wear is often mistaken as stuck calipers More probable root causes are incorrect linings excessive brake duty or incorrect repairs when last serviced See bac
11. ps 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 e Retighten the tube nut e The problem is probably with the pedal linkage e Retighten the tube nut e The problem is probably with the booster NOTE Use appropriate brake component and vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 13 BOSCH Master Cylinder Cap Diaphragms DIAPHRAGM SWOLLEN DUE TO EXPOSURE TO MINERAL OIL POWER STEERING FLUID AUTOMATIC NORMAL DIAPHRAGM TRANSMISSION FLUID MOTOR OIL ETC Figure 4 Comparison Normal vs Swollen Diaphragm NOTE Use appropriate brake component and vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 14 BOSCH Leakage 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 Continued Caliper on next page Is leak at the bleed screw See ABS or vehicle manufacturer s No Yes service manual Tighten bleed screw to manufacturer s recommended torque Does it still leak No Is leak at the brake hose tube fitting Tighten
12. t Often these first simple steps will be enough to determine what repair needs 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 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 4 BOSCH Hydraulic Brake System Diagram MONITORING DEVICE THAT CONTROLS WARNING LIGHT AND BUZZER HYDRAULIC BOOSTER MASTER CYLINDER POWER STEERING RESERVOIR POWER STEERING PUMP a ir iS POWER STEERING GEAR BOOSTER BACKUP PUMP ABS HYDRAULIC CONTROL UNIT BRAKE CALIPER Figure 1 Brake System Components NOTE Use a
13. tion exists The problem is probably with the brake light switch Is there voltage to the relay coil The problem is probably with The problem is stobably with Check for proper power The problem is the booster ihe ignid n steering pump flow by probably with backup pump auch rapidly rotating the steer the booster relay ing wheel the alterna backup pump tive is to measure the relay fluid flow The problem The problem is is probably probably with with the power the booster flow steering pump switch NOTE Use appropriate brake component and vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 21 BOSCH Brake Warning Light and Buzzer Do Not Shut off 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 Is the master cylinder differential with appropriate type brake fluid pressure switch closed to ground No Yes Are light and buzzer still on See Booster Backup Pump Runs Continuously section or Booster Backup Pump Is there a leak in the brake fluid system No Yes Check brake fluid system for leakage per Leakage section Check s
14. tube fitting to manufacturer s recommended torque Replace tube seat or bleed screw Tighten Does it still leak to manufacturer s recommended torque Replace tube seat Does it still leak or fitting Tighten to manufacturer s No a The problem is ae as probably with the caliper Yes No The problem is probably with the caliper NOTE Use appropriate brake component and vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 15 BOSCH Leakage continued Brake fluid mixes with water Power steering fluid floats on water From previous page Examine booster and master cylinder assembly Is leak at booster to master cylinder interface See Figure 3 No Is leakage at booster or Determine if the leak is master cylinder brake fluid or power steering Booster Master Cylinder fluid Brake Fluid Power Thoroughly clean Steering Fluid the entire booster with soap and water then dry it Thoroughly clean the entire booster 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 Can the leakage be fixed by tightening the tube nuts or replacing the tube seats Tighten tube nuts Replace master to manufacturer s cylinder recommended torque or replace tube seat as needed Continued on next page with soap and water then dr
15. ure from the power steering pump 2 The ABS hydraulic unit is blocking the flow of brake fluid to the calipers 3 Binding pedal linkage the brake pedal lever must be aligned side to side throughout full pedal travel such that the booster pedal rod is aligned with the booster centerline to prevent linkage binding With the pedal rod in the released position the pin that attaches the booster pedal rod to the brake pedal lever should rotate freely by hand Optimal performance occurs when the booster pedal rod articulates at the smallest angle possible With engine OFF does brake pedal feel very hard No ves e Check for proper power steering Does the backup pump flow and pressure pump run If the steering wheel cannot be rotated rapidly or if flow is measured and found to be low Sas Booster Back Does the booster backup the problem is probably with the up Pump Does Not pump sound like it slows power steering pump R n section down when pushing harder on the pedal The problem is probably The problem with the booster backup is probably pump with the ABS NOTE Use appropriate brake component and vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 18 BOSCH Brake Pedal Continues to Fall with Steady Foot Force If no external leakage the problem is due to bypassing fluid either inside the master cylinder or inside the ABS hydraulic unit
16. urned 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 No Yes Disconnect booster flow switch connector The problem is Check continuity from relay to connector and probably with the Check continuity through flow switch booster backup pump Is wire continuity OK No Yes The problem is probably with the Is the flow switch continuity OK wire The problem is probably with the booster flow switch Is there voltage to the relay coil Check ignition relay circuit and brake light switch to relay circuit for continuity and properly functioning switches Is there battery voltage to the relay 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 and vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 20 BOSCH Booster Backup Pump Runs Continuously 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 condi
17. y 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 Continued on next page NOTE Use appropriate brake component and vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 16 BOSCH Leakage continued From previous page Is booster flow switch leaking No Is leak at booster inlet port The problem is probably with the flow switch O ring seal No Is leak at booster backup pump The problem is probably with the booster inlet fitting O ring seal Is leak at the interface The problem is probably with the between the booster housing backup pump 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 Clean and examine all sealing surfaces probably with the and seal grooves at backup pump booster booster housing and adapter Are surfaces and grooves flat and free of damage The problem is The problem is probably with the probably with the damaged component seals NOTE Use appropriate brake component and vehicle manufacturer s service manual for all repair work Class 5 to 7 Truck amp Bus Hydraulic Brake System Diagnostic Guide 17 BOSCH Brake Pedal Feels Very Hard The most common reasons for a very hard brake pedal are 1 Insufficient flow or press
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