HydroMax Hydraulic Brake Booster Manual

Hydro-Max™ Hydraulic Brake Booster and Master Cylinder

Technical Manual



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.

Correspondence concerning this manual should be addressed to: Robert Bosch LLC Attn A ttn:: Medium Truck Brake Engineering 3737 Red Arrow Highway St. Joseph, Michigan 49085

This manual can be viewed or downloaded at: http://www.bosch.us

Fax: 269-428-6704

August 2003

Hydro-Max™ Technical Manual

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Table of Contents 1 General Description.............. General Description.......................... ........................ ........................ ....................... ....................... ........................ ........................ ........................ ...................... ................ ......

Hydro-MaxTM Booster Booster............ ........................ ....................... ....................... ........................ ........................ ........................ ....................... ............. ..1 Master Cylinder......................................................................................................... 1

Principle of Operation........................................................................................................................ Operation........................................................................................................................ 3 Hydro-MaxTM Booster Booster............ ....................... ....................... ........................ ........................ ........................ ....................... ....................... .............. ..3 Master Cylinder......................................................................................................... 5 Electrical Operating System...................................................................................... 7 Basic Operation of Booster and Master Master Cylinder......................................................8 9 Electronic Monitor Electronic Monitor Module.............. Module......................... ....................... ........................ ........................ ........................ ....................... ............... ....

Application Information............... Information........................... ........................ ........................ ....................... ....................... ........................ ........................ ........................ .................... ........13 Hydraulic Fluids............. Fluids......................... ........................ ....................... ....................... ....................... ....................... ........................ ..................... .........13 Brake Fluid Brake Fluid Tubing, Hoses and Fittings........................... Fittings............... ....................... ....................... ........................ .................. ......13 Booster Fluid Fluid Tubing, Hoses and Fittings........................... Fittings................ ....................... ........................ ........................ .............. ..14 Booster Fluid Booster Fluid Source..................................................................................................15 Installation Recommendations............... ............ ....................... ....................... ........................ ........................ ..................... .........16 Pedal Return Return Springs................................................................................................. 17 Pedal Stop Pedal Stop and Proper Spring Proper Spring Load....................... Load........... ........................ ....................... ....................... ........................ ................ ....18

Trouble Shooting.................................................................................................................................. 19

Printed in the United States of of America America © 2002 Robert Bosch Corp. All rights reserved.


3

General Description The Hydro-MaxTM is a hydraulically hydraulically powered powered brake brake booster booster which which provides provides power power assist assist for applying hydraulic hydraulic brakes. brakes. A booster booster combined combined with a master master cylinder cylinder (refer (refer to to Figure 1) forms the hydraulic brake hydraulic brake actuation unit. A typical assembly is shown in Figure 1. The The booster booster reduces reduces the pedal effort required to apply the the brakes brakes as compared to a non-power non-power system. system.

BRAKE FLUID RESERVOIR

FLOW SWITCH INLET PORT

PEDAL ROD

OUTLET PORT

MOUNTING FLANGE

MASTER CYLINDER HYDRO-MAXTM BOOSTER SECONDARY OUTLET PORT BACKUP PUMP

DIFFERENTIAL PRESSURE SWITCH RELAY

FLUID LEVEL INDICATOR SWITCH PRIMARY OUTLET PORT

Figure 1 Hydro-MaxTM

Booster and Master Cylinder Assembly

Hydro-MaxTM Booster The hydraulic hydraulic booster booster is is comprised of of an an open center center valve valve and reaction feedback feedback mechanism, mechanism, a power piston, piston, a 12 and 24-volt 24-volt backup backup pump pump and an integral flow switch. It is is powered powered by by either the power the power steering steering pump pump or or other other hydraulic hydraulic source. The The backup backup pump pump provides provides a secondary secondary power power source for for the the hydraulic hydraulic booster booster and and is controlled controlled by by the integral flow switch.

Master Cylinder The master master cylinder cylinder is is a split system type with separate fluid chambers, chambers, pistons pistons and outlet outlet ports ports for the front and rear brake brake circuits. A differential differential pressure pressure switch, fluid level indicator indicator switch, switch, and remote reservoir reservoir are are also available. A typical Hydro-MaxTM booster booster and and master master cylinder cylinder assembly assembly installation in a hydraulic hydraulic braking braking system is shown in Figure 2.


41

D N U O R G

R E Z Z U B G N I N T R H A G I W L O G T N I N R A W O T

N O I T I N G I E O L T ) U L D A O N M O I R T P O T O I ( N O M

P D I O T

H C T I W S E K A R B

Y R E T T A B O T

S E K A R B R A E R O T

W O L F F O N O I T C E R I D

M E T S Y S G N I N R A W O T

R A E G G N I R E E T S

M E T S Y S G N I N R A W O T

R E T L I F & R I O V R E S E R

S E K A R B R A E R O T

T I N U S B A

S E K A R B T N O R F O T

S E K A R B T N O R F O T

m e t s y S g n i k a r B c i l u a r d y H a n i n o i t a l l a t s n I y l b m e s s A r e d n i l y C r e t s a M d n a r e t s o o B l a c i p y T 2 e r u g i F

P M U P G N I R E E T S


52

Principle of Operation Hydro-MaxTM Booster During normal system operation, (refer to to Figure Figure 4) fluid flow from a hydraulic power source (usually the power steering steering pump) pump) enters the inlet inlet port port of of the the Hydro-MaxTM booster, flows through the the power power piston, piston, around the throttle valve and through the flow switch, exiting through the outlet outlet port. port. Force applied to the the brake brake pedal pedal by the vehicle operator is multiplied by the lever lever ratio ratio of of the the pedal pedal mechanism to move the the pedal pedal rod of of the the booster. booster. This movement closes the throttle valve, which restricts flow. This restriction of of flow, flow, which results in a pressure increase acting on the the power power piston, piston, applies an amplified force to the master master cylinder cylinder primary primary piston. A reaction piston, inside the the power power piston piston subassembly, provides subassembly, provides the driver driver "pedal "pedal feel" during an application of of the the brake brake pedal. pedal. Fluid flow through the flow switch opens the the backup backup pump pump electrical circuit during normal operation. A separate check check valve valve in the the backup backup pump pump prevents prevents back-flow through the the pump pump during normal normal power power applications. In the event normal flow from the the power power source is interrupted, the the backup backup pump pump provides provides the the power power at at a reduced rate for stopping. See See Figure Figure 3 for performance performance curve. Upon flow interruption, the integral flow switch closes, energizing a relay, providing electrical power power to to the the backup backup pump. pump. During backup operation, the During backup the pump pump re-circulates fluid within the the booster booster assembly with with pressure pressure built on demand via the throttle valve. Fluid is retained within the the booster booster by by the inlet inlet port port check check valve. valve. Figure 3 illustrates the typical relationship of of master master cylinder cylinder pressure pressure and input force of the the booster. booster.

Figure 3

Typical Performance Curve

Hydro-Max™ Technical Manual 3 6

Y ’ S S A H C T I W S W O L F

G N I R P N S O W T S O I L P F H W C T O I L W F S W O L F

Y S S A H ) C N T E I P W O S (

G D N O I R R L G T P A N F S D I A R E H H O P S S T U T A G B U L & N P U I G N S R L I E O A N R E O E & S T T S R I U A T F E A P S S H R S E E L L S E R V A W T P L O A B U P P K T V C N R K E I Y O C E H S T P H C S R T A O C E ) P H D L N C E T R E I T S E T I L L W O T I S L U F C ( O W O L F

T E M M O R G

N I P N O I T C A E R

N O T S I P N O I T C A E R

E V L A N V O E T S L I T P T R O R E H W T O P

G N I R P S N R U T E R E V L A V

G N I R P S N R U T E R N O T S I P R E W O P

L L A B K C E H C P M U P P U K C A B

W O L F

P A C D N E

T F A H S T U P T U O N O T S I P R E W O P

r e d n i l y C r e t s a M d n a r e t s o o B M T

x a M o r d y H 4 e r u g i F


7 4

Master Cylinder In the released released position, position, (refer (refer to to Figure Figure 5) actuators of both both the the primary primary and secondary secondary pistons pistons are in with their respective their respective compensating valve stems, which which project project into the cylinder bore. bore. This contact tilts the valves to an open open position, position, which allows hydraulic fluid in the reservoir reservoir sections sections to communicate with the the primary primary and secondary secondary pressure pressure chambers. Each Each pressure pressure chamber chamber has has a piston/actuator piston/actuator subassembly subassembly containing a preloaded (caged) spring and return spring. Initial forward travel of of the the primary primary piston piston moves the the primary primary actuator actuator away away from its compensating valve, permitting the valve to seat. Closure of of this this valve shuts off off the the passage passage between between the the primary primary pressure pressure chamber and chamber and the reservoir reservoir section section serving the the primary primary chamber. Further movement of Further movement of the the primary primary piston piston creates creates pressure pressure in the the primary primary pressure pressure chamber, causing the secondary piston secondary piston and actuator actuator to to move. As the secondary secondary piston piston and actuator actuator move, move, the secondary compensating valve closes, shutting off off the the passage passage between between the secondary secondary pressure pressure chamber chamber and and the reservoir section serving the secondary chamber. Additional movement of of the the primary primary piston piston causes causes both both chambers to build pressure. When the load on the the primary primary piston piston is removed, fluid fluid pressure pressure in each chamber, combined with return spring force, causes the the primary primary and secondary secondary pistons pistons to return to their their initial initial released released positions. positions. Each actuator opens actuator opens its respective compensating valve, reopening the the passage passage between between the individual reservoir sections and its associated associated pressure pressure chamber. Should the rate of of release release be be great enough to cause a partial vacuum in the chamber, the compensating valve will open to allow replenishment of of fluid fluid into the cylinder bore. bore. Any excess fluid remaining at the end of of the the stroke due to "pumping" and/or and/or volume volume change due to temperature fluctuation is released to the reservoir reservoir as as the compensating valves open. The primary circuit is separated from the secondary hydraulic circuit. Hydraulic leakage in one circuit does The primary not affect the function of of the the other other circuit. circuit. A fluid level indicator indicator switch switch is available. It illuminates a light on the dash dash panel panel to warn of of low low brake brake fluid level in the master master cylinder cylinder reservoir. reservoir. A low fluid level can result from from brake brake shoe lining wear, or or it it can occur if occur if there there is an external leak leak in in the vehicle vehicle brake brake system. A differential differential pressure pressure switch is available. It illuminates a light on the dash dash panel panel to warn when there is a pressure differential differential between between the the primary primary and secondary secondary brake brake circuits caused caused by by a leak leak in in one circuit. This may occur occur when when one circuit leaks or or is is improperly improperly bled. bled. A remote reservoir reservoir application application is available where under-the-hood space constraints constraints prohibit prohibit the use of of aa conventional booster conventional booster and and master master cylinder cylinder assembly. assembly. In a remote application, the master master cylinder cylinder reservoir reservoir is is mounted separately from the master master cylinder. cylinder.

Hydro-Max™ Technical Manual 58

r e d n i l y C r e t s a M 5 e r u g i F


9 6

Electrical Operating Systems

Figure 6 Schematic of Electrical Electrical Operating Operating System


Basic Operation of Hydro-MaxTM Booster Assembly and Master Cylinder Reference Figure Reference Figure 6 6 for for the the following electrical components of of the the booster booster and and master master cylinder. cylinder. 1. Backup Pump: The Hydro-MaxTM hydraulic hydraulic booster booster has has a backup backup pump pump which will will provide provide hydraulic boost hydraulic boost at a reduced rate if if the the normal source of of fluid fluid is interrupted (refer (refer to to Figur Figuree 3). The signal for for operation operation of the the backup backup pump pump comes from the flow switch. If If normal normal flow is interrupted, the flow switch will close and activate the relay, which will turn on the the backup backup pump. pump. The The backup backup pump is available for for 12 12 and 24-volt systems. The 12-volt 12-volt backup backup pump pump can draw a steady state maximum of of 55 55 amps at a power power steering steering fluid Ο temperature of of 100 100 F. The 24-volt 24-volt backup backup pump pump can have a steady state maximum draw of of 27.5 27.5 Ο amps at 100 F. 2. Relay: An optional relay is available for for attachment attachment to the the booster booster.. The relay connects to a sealed and mechanically latched wiring connector. The function of of the the relay is to to provide provide current to the backup pump backup pump when triggered triggered by by the flow switch. 3. Flow Switch: The function of of the the flow switch is to activate the relay when normal hydraulic hydraulic power power source fluid flow is interrupted, turning on the the backup backup pump. pump. The flow switch has two terminals (A and B). A is is positive. positive. B is connected to the the booster booster housing housing and can can be be used as an optional harness ground. The flow switch itself itself is is grounded through the the booster booster housing. housing. A sealed and mechanically latched connector connector connects connects the flow switch to the the backup backup pump pump relay circuitry. 4. Differential Pressure Switch: The differential differential pressure pressure switch reacts to a loss of of master master cylinder cylinder hydraulic pressure hydraulic pressure in either either side side of of the the split hydraulic system and can illuminate a warning light at the instrument panel. instrument panel. When When pressure pressure is lost in either either the the primary primary or or secondary secondary circuit of of the the master cylinder, the switch closes. The electrical switch will remain closed until the malfunction is corrected. When both When both systems develop normal normal pressure, pressure, the switch will return to center center and and open the electrical switch circuit. A mechanically latched connector connector is is attached to the switch. The switch has two tabs on a common terminal (positive) and is grounded through the master master cylinder. cylinder. 5. Fluid Level Switch: When the fluid reaches a predetermined level (low fluid level), the switch closes and can illuminate a warning light at the instrument instrument panel. panel. A sealed and mechanically latched connector is connector is attached. The switch has two terminals, terminals, positive positive and negative, which are interchangeable. 6. Chassis Ground : The The booster booster and and master master cylinder cylinder assembly assembly must must be be provided provided with a ground ground path path that will carry the maximum current of of the the backup backup pump. pump.


8 11

Electronic Monitor Modules

(12-volt only)

Bosch recommends that a monitor monitor module module be be included to monitor monitor the the electrical integrity of the backup the backup system. Bosch offers two types with the following features and functions:

Monitor Module Type I Features 1. Pe Perf rfor orms ms start-up self self test test for proper electrical proper electrical signal to reserve reserve backup backup pump. pump. 2.

Cont Co ntin inuou uousl sly y test for for an an open circuit in in backup backup pump. pump.

3. Tur urns ns on vehicle warning light and and buzzer buzzer if if any any of of the the above tests indicate a backup system electrical malfunction. The warnings are continuous until the malfunction is corrected or or the the power is power is shut off. 4. Co Conv nvey eyss signal to to provide provide electrical electrical power power to to the the backup backup pump pump via the relay. 5. Pr Prov ovid ides es a centralized housing for for diodes. diodes. 6. In Incl clud udes es an integral differential differential pressure pressure switch light check check at at start up.

Figure 7 Schematic of Electronic Monitor Monitor Module Type Type I


9 12

Pin Functions Pin

Function

1

a) Provides Provides path path to flow switch to ground for pin pin 2 to allow test of of red red light during vehicle start up. b) Suppresses negative voltage transients into into pin pin 5 from the relay coil.

2

Provides path to to pin pin 1 to flow switch to vehicle ground to allow test of of red red light during vehicle start up. Note: Pin 1 and 2 are connected, with a diode in-between. With the key in the "on" position, the red dash dash panel panel light should illuminate anytime hydraulic flow (from the fluid power fluid power source) source) drops to a range of of 0.5 0.5 to 1.1 gpm. (The range at which flow switch continuity occurs).

3

Open circuit when all is okay, okay, but but switches to ground (pin 4) during vehicle start or or during during a detected fault. Connection to ground turns on warning light and and buzzer. buzzer.

4

Connects monitor monitor module module to vehicle ground.

5

Supplies power power from from module to amber amber light, light, buzzer buzzer and and relay coil. Current flow through this terminal must must be be limited to 300 mA.

6

a) Monitors for for voltage voltage to to backup backup pump pump during startup. If If none none detected, detected, pin pin 3 remains grounded (connected to to pin pin 4). b) Monitors for for continuity continuity through the the backup backup pump pump to vehicle ground during vehicle operation (engine on). Pin 3 switches to ground (pin 4) if if continuity continuity is lost.

7

Brings power power into into module from ignition switch.

8

Brings power power into into module from from brake brake switch. Only needed when ignition switch is off. Enables backup Enables backup pump pump to operate when ignition is off.


13 10

Monitor Module Type II

(12 volt only)

Features 1. Per Perfo form rmss start-up self self test test for proper electrical proper electrical signal to to backup backup pump. pump. 2. Cont Continu inuous ously ly tests for for an an open circuit in in backup backup pump. pump. 3. Tes ests ts for for voltage voltage during during brake brake application in in brake brake light circuit and and backup backup pump pump relay coil circuit on vehicles equipped with dual dual brake brake switches. 4. Tur urns ns on warning light(s) and and buzzer buzzer if if any any of of the the above tests indicate a backup backup pump pump system electrical malfunction. The warnings are continuous until the malfunction is corrected or or the the power is power is shut off. 5. Ab Abse senc ncee or or malfunction malfunction of of the the module will not affect affect backup backup system operation. 6. Tur urns ns on light(s) and and buzzer buzzer when when integral differential differential pressure pressure switch is activated.

Figure 8

Schematic of Electronic Electronic Monitor Module Type II


11 14

Pin Functions Pin

Function

A

Suppresses negative voltage transients into into pin pin E from the relay coil.

B

Not used.

C

Open circuit when all is okay, okay, but but switches to ground (pin D) during vehicle startup or during a detected fault. Connection to ground switches on warning light and and buzzer. buzzer.

D

Connects monitor monitor module module to vehicle ground.

E

Power input input to monitor monitor module module from ignition switch or or Hydro-Max Hydro-MaxTM brake brake pedal pedal switch.

F

1) Monitors for for voltage voltage to to backup backup pump pump during vehicle startup. If If none none detected, detected, pin pin C remains grounded (connected to to pin pin D). 2) Monitors for for continuity continuity through through backup backup pump pump to vehicle ground during vehicle operation (engine on). Pin C switches to ground (pin D) if if continuity continuity is lost.

G

Monitors output of of the the backup backup pump pump switch that turns on the the backup backup pump, pump, to allow comparison with with pin pin H. If If different different for for > > 14±5 seconds, then then pin pin C switches to ground (pin D) and latches.

H

Monitors output of of stop stop light switch to allow comparison with with pin pin G. If If different different for for > > 14±5 seconds, then then pin pin C switches to ground (pin D) and latches.

J

Not used.

K

Not used.


12 15

Application Information Hydraulic Fluids The Hydro-MaxTM booster booster uses uses power power steering steering fluid as the medium of of transmitting transmitting power. power. The master master cylinder cylinder uses uses DOT 3 brake fluid fluid per per SAE SAE J1703, unless otherwise specified on the top of the of the reservoir. Service maintenance of of motor motor vehicle vehicle brake brake fluid in motor motor vehicle vehicle brake brake actuating systems is covered in SAE J1707 information report.

Before using any other type of fluid in the booster or in the master cylind er, contact the Bosch Hydraulic Actaution and Truck Brake Engineering Department. (See inside of front cover).

The booster and the master cylinder use two (2) distinctly different incompatible hydraulic fluids. fluids. They must not be mixed. Using the incorrect incorrect fluid will permanently damage the seals and can cause the brakes to malfunction.

Brake Fluid Tubing, Hoses and Fittings Brake fluid lines (tubing and flexible hose) transmit fluid under pressure between pressure between the master cylinder and cylinder and the the brakes. brakes. The hoses are the flexible links links between between wheels or or axles axles and the frame or body. The hoses must withstand fluid fluid pressure pressure with minimal expansion and must must be be free to flex without damage during normal suspension deflection and wheel turns. The following SAE specifications, or or their their successors successors if if appropriate, appropriate, are recommended for for consideration consideration as a minimum requirement when outfitting a vehicle with with brake brake fluid tubing, hoses and fittings. The hoses should conform to SAE J1401 Hydraulic Brake Hose - Automotive. The tubing should conform to SAE J1047 Tubing - Motor Motor Vehicle Vehicle Hydraulic Brake System. The fittings should conform to SAE J516a - Hydraulic Hose Fittings, or or SAE SAE J512 - Automotive Tube Fittings. Contact the vehicle manufacturer manufacturer for for specific specific requirements for for individual individual applications.


13 16

Booster Fluid Tubing, Hoses and Fittings The pressure line that supplies fluid to the The pressure the booster booster must must be be a 1/2" diameter diameter flexible flexible or or rigid rigid pressure pressure line conforming to SAE J188 and typically designed to run from the steering gear gear to to the HydroTM TM Max inlet. The Hydro-Max inlet inlet port port utilizes a tube "O" arrangement as shown in in Figure Figure 9 (not supplied supplied by by Bosch). For For tube tube flare configuration see see Figure Figure 10.

MINIMUM THREAD

TUBE AND NUT ASSEMBLY 2771464 (TIGHTEN TO 22 - 34 Nm)

NOT SUPPLIED BY BOSCH

O-RING 2772007

CHECK VALVE SEAT & O-RING CHECK BALL

Figure 9 Tube"O" Fitting and O-ring Seal

TUBE O.D.

Figure 10 Tube Flare Configuration

The return line must must be be a 1/2" diameter diameter hydraulic hydraulic hose attached to the Hydro-MaxTM outlet outlet port port (See Figure (See Figure 11). The return line and clamp must must be be selected so that it will withstand worst case vehicle operating conditions, such as as back back pressures pressures resulting from start up at extremely cold temperatures or or from from devices installed down stream of of the the booster. booster.


17 14

Figure 11 Outlet Port

Booster Fluid Source The booster is The booster is designed to to perform perform in a flow range of of 3.2 3.2 to 5.0 gpm. Flow rate through the booster can booster can impact vehicle stopping distance; therefore, the vehicle manufacturer manufacturer must must determine the required minimum flow rate. Flow rates rates below below 3.2 gpm can cause slower slower response response times when the booster the booster is is applied. Flow rates higher higher than than 5.0 gpm may contribute to the the booster booster self-applying, self-applying, which will cause cause brake brake drag or or fluid fluid overheating. The fluid fluid power power source source must must be be capable of of supplying supplying 1,000 1,000 psi psi to the the booster booster plus plus any additional pressure required required by by other other devices devices in the system such as a power power steering steering gear. If a power If a power steering steering gear gear is is used, the steering gear gear must must be be "balanced" so that it can handle the pressures generated in the steering gear gear return return line. It must also have an internal relief relief valve valve setting lower lower than than the the pump pump relief relief to to allow the steering gear gear to to relieve relieve before before the hydraulic hydraulic pump pump does. It must also have an internal internal by-pass by-pass to allow manual steering during during booster booster backup backup system operation. The manufacturer manufacturer must must adhere to the flow flow path path illustrated in in Figure Figure 2.


Installation Installati on Recommendations The Hydro-MaxTM booster booster is is generally mounted to a vehicle's dash dash panel. panel. If If necessary, necessary, the HydroMaxTM booster booster and and master master cylinder cylinder may may be be mounted remotely from the vehicle dash dash panel panel and actuated by actuated by appropriate linkage. Four mounting holes are Four mounting are provided provided in the the booster booster housing housing through which up to four four studs studs can can be be pressed. Use of of (Grade (Grade 5) 3/8-16 or or M10 M10 locknuts, having self-contained self-contained prevailing prevailing torque features that are frictionally resistant to rotation, are recommended. See Figure 12 for See Figure for the the three three basic basic pedal pedal rod designs offered. The threaded threaded pedal pedal rod screws into the customer supplied customer supplied attachment to the the brake brake pedal. pedal. The The pin pin that attaches the the pedal pedal rod to the the brake brake pedal must insert freely. The attachment mechanism of of the the pedal pedal rod should should be be of of sufficient sufficient strength to prevent to prevent linkage linkage binding. binding.

THREADED

SINGLE EYE

CLEVIS

Figure 12 Basic Pedal Rod Designs


16 19

ATTENTION Regardless of of mounting mounting location, the following requirements must be followed: a. Moun Mountt the unit such that the the pedal pedal rod, throughout full full pedal pedal travel, does not articulate more than than plus plus or or minus minus 5Ο above or below booster below booster centerline. centerline. The The brake brake pedal lever lever must must be be aligned side to side with the the booster booster centerline centerline to to prevent prevent linkage binding. linkage binding. b. The brake The brake pedal pedal and mechanical linkage must provide enough travel to fully stroke both booster both booster and and master master cylinder, cylinder, i.e., the the brake brake pedal pedal must not not bottom bottom on the floor of the of the vehicle. A pedal ratio of between between 2.8:1and 3.3:1 is recommended to to provide provide the desired "pedal feel". c. The Hydro-MaxTM booster booster and and master master cylinder cylinder assembly assembly must be mounted horizontally. d. The reservoir reservoir of of the the master master cylinder cylinder must be upright. The The backup backup pump pump must always be always be at the the bottom bottom of of the the booster booster.. e. The unit must be be protected protected from environmental and fluid temperatures over over 250 250ΟF.

General Safety Note Unique usage or or applications applications of of the the Hydro-MaxTM booster booster and and master cylinder master cylinder assembly assembly requires input from Bosch and validation by validation by the vehicle manufacturer.

Pedal Return Springs Any vehicle application utilizing a Hydro-MaxTM booster booster and and a brake brake pedal pedal return spring must have: 1. A properly adjusted adjusted pedal pedal stop 2. A limited spring load at the the pedal pedal released released position. position.


20 18 17

Pedal Stop and Proper Spring Load The pedal stop must The pedal must be be adjusted such that when the the brake brake pedal pedal is fully released, there is no load on the grommet that retains the the pedal pedal rod to the Hydro-MaxTM booster booster.. When the the brake brake pedal pedal is fully released, the spring load must not not be be greater greater than than that required to support the the pedal pedal weight. The reason for for aa properly adjusted adjusted pedal pedal stop and for for the the limitation on spring load is to to prevent prevent the spring from from pulling pulling on the grommet within the Hydro-MaxTM booster booster and and damaging the grommet. The grommet is not designed to to be be loaded in this manner. Any adjustment of of the the brake brake switch and and backup backup pump pump must not compromise the adjustment of of the the pedal stop.

It is a commmon practice to stack vehicles on top of each other to deliver the vehicles. An air cylinder is typically used to apply the brake pedals in the towed vehicles. When installing installing such a cylinder, be sure that the backup pump turns turns off when the towing vehicle brake pedal is released.


21 18

TROUBLE SHOOTING Condition Hard Pedal

Excessive pedal travel (pedal goes to the floor)

Cause

Remedy

Binding pedal Binding pedal linkage

Contact dealer dealer for for service service

Pedal misaligment (side to side)


Low booster Low booster supply supply flow

Repair/replace power steering Repair/replace power steering pump, check check tension tension in in belt belt

Power steering pump Power steering pump flow control valve

Repair/replace power steering Repair/replace power steering pump, check check tension tension in in belt belt

Power steering steering pump pump relief valve setting too low

Replace power Replace power steering steering pump pump

Fluid contamination

See vehicle service manual for type of of fluid fluid required. Drain, flush and refill system, replace booster or booster or master master cylinder cylinder as as appropriate, and and power power steering steering reservoir filter reservoir filter as as necessary

Master cylinder Master cylinder

Replace master master cylinder cylinder

Booster

Replace booster Replace booster

Tube and hose hose passages passages are blocked shut

Replace parts containing Replace parts blocked passages blocked passages

Improper pedal pedal adjustment

See vehicle service manual for pedal pedal rod adjustment (if system (if system is adjustable)


TROUBLE SHOOTING Condition

Cause

Remedy

Excessive pedal Excessive pedal travel (pedal goes to the floor)

External leak leak in in brake brake fluid system

Check all fittings and Check all connections for for the the master cylinder, calipers, hoses, and tubes, replace if if necessary necessary

(continued)

Internal master master cylinder cylinder leak


Air in Air in brake brake fluid system

Bleed system

Brake linings

Inspect and adjust, or or replace replace brake shoes, if if necessary necessary



Weak brake brake hoses that expand under pressure pressure

Replace hoses

Poor quality brake Poor quality brake fluid (low boil (low boil point) point)

Drain the system, flush and refill with the recommended fluid.

Brake self self adjusting adjusting mechanism

See vehicle service manual (if brakes are adjustable)

Air in hydraulic Air in hydraulic brake brake system

Bleed brake Bleed brake system

Poor quality brake Poor quality brake fluid (low boil (low boil point) point)

Drain the system, flush and refill with recommended fluid

Weak brake brake hoses that expand under pressure pressure

Replace defective hoses



Pulsating brake pedal brake pedal

Soft, spongy brakes

Actuated pedal Actuated pedal does not return


23 20

TROUBLE SHOOTING Condition

Cause

Actuated pedal Actuated pedal does not return

Fluid contamination




Binding booster Binding booster


Self applied booster Self applied booster (fluid flow too high)

Repair or replace Repair or replace power power steering pump. steering pump.

ABS operation

See appropriate service manual

Wheel bearings loose or Wheel bearings worn


Excessive rotor rotor hickness hickness variation


Parking brake control valve Parking brake (HR1) has internal leak


Self applied booster Self applied booster (fluid (fluid flow too high)

Repair or replace Repair or replace power power steering steering pump

Constricted master master cylinder cylinder reservoir vent reservoir vent

Clean vent vent passage passage in reservoir cap

Self applied booster Self applied booster (fluid (fluid flow too high)

Repair or replace Repair or replace power power steering pump

(continued)

Pulsating brake Pulsating brake pedal

Brake self self apply apply

Dragging brake Dragging brake

Remedy


24 21

TROUBLE SHOOTING Condition Dragging brake Dragging brake (continued )

Noise / Whistle

Backup pump Backup pump operates continuously

Cause

Remedy

Stuck or bound Stuck or bound caliper pins or or rails rails


Binding booster Binding booster






Soft or or swollen swollen master cylinder seals cylinder seals due to to brake brake fluid contamination

Flush out old old brake brake fluid then replace master master cylinder cylinder and and add new brake new brake fluid

Tube and hose hose passages passages restricted

Replace the the parts parts containing the restriction

Inherent in some some boost boosters (whistle)

None - Does not affect operation

Brake switch

See vehicle service manual

Monitor module Monitor module


Relay


Flow switch

Remove switch components, clean, inspect for for damage damage or contamination and replace if necessary




25 22

TROUBLE SHOOTING Condition Backup pump Backup pump does not operate

Leakage

Cause

Remedy

Same as above

Repeat as above

Electrical ground

See service manual

Backup pump Backup pump

Replace backup Replace backup pump pump



Booster


Master cylinder reservoir Master cylinder reservoir vent caps

Replace diaphragms and caps

Fluid contamination


Backup pump Backup pump mounting seals

Replace seals

Flow switch seal

Replace o-ring

Hydraulic fittings and connections

See vehicle service manual, tighten and replace if if necessary necessary

Silicone floating This material is an assembly in master master cylinder cylinder lubricant

None - Does not affect operation


26 23

TROUBLE SHOOTING Condition Fluid level indicator light indicator light comes on

Cause

Remedy

Low fluid level

Inspect brake pads, Inspect brake pads, replace as necessary, add fluid as required

Fluid level indicator indicator switch switch


Brake pads Brake pads worn out

Inspect brake pads, Inspect brake pads, replace as necessary, add fluid as required

Brake indicator Vehicle specific light and and buzzer buzzer malfunction



27 24

Correspondence concerning this manual should be addressed to:

Robert Bosch LLC Medium Truck Brake Engineering Medium Truck Engineering 3737 Red Arrow Highway St. S t. Joseph, Michigan 49085

Fax: 269-428-6704

Printed in the United States of of America America Copyright © 2002 Robert Bosch Corporation

HydroMax Hydraulic Brake Booster Manual

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