BENDIX BW1399 User Manual

TROUBLESHOOTING THE

VACUUM HYDRAULIC BRAKE

SYSTEM

Troubleshooting The

Vacuum Hydraulic Brake System

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 rela­tively short period of time by performing these tests.

While in many cases a problem can be found and cor­rected early in the testing procedure, it is generally rec­ommended that all three phases of testing be completed
to assure that the entire system is in good working order.

BEFORE GETTING STARTED

When working on or around brake systems and compo­nents, 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 prop­er tools and observe all precautions pertaining to use
of these tools.

5. Use only genuine Bendix replacement parts and

components.

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 per­formed 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.

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 gaso­line, 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 profes­sional advice.

DATE:

TRUCK

MECHANIC:

PHASE I – VISUAL
INSPECTION

GENERAL

Phase I visual inspection procedures should reveal the most com­mon problems found in a hydraulic brake system. The inspection
steps are presented in the order in which they are most easily
performed.

Brake Fluid Boiling Point Boiling Point

Type Dry Wet

DOT #3 401°F 284°F

DOT #4 446°F 311°F

DOT #5 500°F 500°F

(silicone)

With the master cylinder cover off, have an assistant slowly make

a full brake application while observing the fluid in the reservoir.

CAUTION. Wear eye protection and do not allow fluid to contact

vehicle's painted finish. A small spurt or geyser at the beginning of

the brake application followed by continued fluid turbulence indi-

cates internal master cylinder leakage. Repair or replace. If the

fluid remains calm after initial spurt, continue to Phase II Testing.

PHASE II – HYDRAULIC
SYSTEM TESTS

INSPECTION

1. Look for signs of fluid leakage at connection points as well as
around wheel cylinder calipers, the master cylinder and
hydrovac. Inspect disc brakes caliper piston boots for tears or
deterioration.

2. Inspect the master cylinder pedal and linkage for binding, bend­ing or damage. Lubricate rotating and sliding components with
a rubber compatible lubricant.

3. Inspect the foundation brakes for proper adjustment and adjust
if necessary.

4. Start engine and have assistant make several brake applica­tions while visually inspecting all hydraulic and vacuum lines,
hoses, and clamps for kinking, chafing, damage, corrosion and
signs of collapsing or "ballooning".

5. Remove the master cylinder reservoir cover and check the fluid
level. Refill as necessary. While the cover is off, look for discol­oration, cloudiness, and "gumming" of the fluid. Also look for
corrosion on any part in contact with the fluid. If any of these
conditions are found, the system should be drained, bled,
flushed and refilled with the proper type of brake fluid for the
application. DO NOT MIX HYDRAULIC FLUID AND BRAKE
FLUID. "GUMMING" OF FLUID AND DAMAGE TO COMPO­NENT SEALS WILL RESULT.

Federal Motor Vehicle Safety Standard #116 divides brake
fluids into three categories with the primary difference being the
fluid boiling points.

GENERAL

1. With the engine off and brakes not applied, install a 0-2000 p.s.i.
hydraulic gauge at the wheel cylinder or caliper and bleed air
from the gauge and its connecting line.

NOTES:

A. The gauge can usually be installed by removing the wheel

cylinder bleed screw.

B. When the vehicle is equipped with a dual or "split" brake sys-

tem, a gauge must be installed in each side of the system,
i.e.; one gauge in a front axle brake and one gauge in a rear
axle brake. Alternatively, the tests can be performed twice,
once for the front axle brakes and again for the rear axle
brakes.