BENDIX BW1399 User Manual

TROUBLESHOOTING THE

VACUUM HYDRAULIC BRAKE

SYSTEM

oubleshooting T

Vacuum Hy

aulic Br

e System

DA
TE:

TRUCK

MECHANIC:

The three phases of tests presented here have been

designed to discover the presence of both vacuum and

hydraulic fluid leakage and sluggish performance of the

brakes. The entire system from the vacuum service to the

wheel cylinder or caliper can be easily checked in a rela-

tively short period of time by performing these tests.

While in many cases a problem can be found and cor-

rected early in the testing procedure, it is generally rec-

ommended that all three phases of testing be completed

to assure that the entire system is in good working order.

To perform these tests it is necessary to obtain both a

vacuum and hydraulic pressure gauge. The hydraulic

gauge should be capable of readings up to 2000 P.S.I.

while the vacuum gauge should have a range up to 30

inches of mercury.

Please note that the tests presented here are per-

formed with the vehicle stationary. They must not be

interpreted as overruling the importance and necessity of

functional dynamic controllability tests and other testing

required in assuring vehicle safety and performance.

When working on or around brake systems and compo-

nents, the following precautions should be observed:

1. Always block vehicle wheels. Stop engine when work-

ing under a vehicle. Keep hands away from chambers;

they may apply as system pressure drops.

2. Never connect or disconnect a hose or line containing

pressure; it may whip. Never remove a component or

pipe plug unless you are certain all system pressure

has been depleted.

3. Never exceed recommended pressure and always

wear safety glasses.

4. Never attempt to disassemble a component until you

have read and understand recommended procedures.

Some components contain powerful springs and injury

can result if not properly disassembled. Use only prop-

er tools and observe all precautions pertaining to use

of these tools.

5. Use only genuine Bendix replacement parts and

components.

A. Only components, devices and mounting and

attaching hardware specifically designed to use in

hydraulic brake systems should be used.

B. Replacement hardware, tubing, hose, fittings, etc.

should be of equivalent size, type and strength as

the original equipment.

6. Devices with stripped threads or damaged parts

should be replaced. Other than drums and rotors,

repairs requiring machining should not be attempted.

7. For cleaning brake parts, use only new DOT 4 brake

fluid, isopropyl alcohol or clear methylated spirits. Do

not use mineral base cleaning solvents such as gaso-

line, kerosene, carbon tetrachloride, acetone or paint

thinner - these will damage the rubber parts.

8. Bendix recommends that for optimum results and

safety, worn units should be replaced by new units. If,

however, it is decided to service a used component,

particular attention should be paid to the following, but

in any case of doubt replace the unit or seek profes-

sional advice.

BEFORE GETTING STARTED