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2-2
G R O U P 2-BRAKES
Q
DIAGNOSIS AND
The trouble shooting symptoms,
causes, and corrections given in the
following diagnosis guide apply to
all truck hydraulic brakes including
those with a vacuum booster.
PRELIMINARY CHECKS-
STANDARD HYDRAULIC
SYSTEM
Push the brake pedal down as f a r
DIAGNOSIS GUIDE—STANDARD
BRAKES D O NOT APPLY
EXCESSIVE PEDAL TRAVEL
UNEVEN, NOISY,
GRABBING, OR HARD
OPERATING BRAKES
BRAKES D O NOT RELEASE
TESTING
as it will go. If the pedal travels
more than halfway between the re-
leased position and the floor, adjust
the brakes.
Road test the truck and apply the
brakes at a speed of about 20 m p h
to see i f the truck stops evenly. If
not, the brakes should be adjusted.
Perform the road test only when the
HYDRAULIC BRAKES
If the brake pedal travels all the
way down to the floor without notice-
able brake action, check the brake
fluid level in the master cylinder res-
ervoir. Refill the reservoir i f neces-
sary. Check the entire hydraulic sys-
tem for fluid leaks, and make the
necessary adjustments.
If the brake pedal feels spongy
when pushed down, air has entered
the hydraulic lines. Air can enter the
lines i f the fluid level in the master
cylinder reservoir is too low, or if the
brake wheel cylinder pistons are not
held firmly in place when the brake
Check for air in the brake lines
and bleed the system i f necessary.
R e m o v e the brake drums so that a
complete inspection of the brake as-
semblies can be made to determine
the cause of the trouble.
Excessive dust and dirt in the
brake lining rivet holes or in the
brake drum can cause brake squeal.
R e m o v e the dirt with a scraper and
an air hose.
D r u m s
which
are out-of-round
or loose at the hub; frozen master
cylinder or brake cylinder piston(s);
defective
check
valve;
improper
Check for an improperly adjusted
brake pedal, a restricted by-pass port
in the master cylinder, or swollen
master cylinder piston cups. Check
for a defective check valve restrict-
ing fluid passing through the system.
Check for sticking brake cylinder
pistons caused by dirty or contami-
nated brake fluid.
Adjust the brake pedal i f neces-
sary. If the adjustment does not cor-
rect the trouble, check the condition
of the brake fluid. Replace dirty or
contaminated fluid. Clean the entire
brakes will apply and the truck can
be safely stopped.
Apply steady pressure to the brake
pedal. If i t moves slowly toward the
floor, check for leaks in the master
cylinder, brake cylinders, or else-
where in the hydraulic system (Fig.
1). If the brake pedal feels spongy,
bleed the system to remove air from
the lines.
shoes are serviced. A defective check
valve can cause a loss of residual
pressure in the system causing air to
enter at the wheel cylinder piston.
Bleed the system to remove air from
the lines, and adjust the brakes. Refill
the master cylinder reservoir with
heavy-duty brake fluid. If the brakes
do not apply after making these
checks and adjustments, fluid m a y
be leaking past the piston cups in the
master cylinder or brake wheel cyl-
inder^). If the trouble is in the mas-
ter cylinder or brake wheel cylin-
der(s), remove and repair.
Adjust
or reline the brakes as
needed.
brake shoe adjustment; warped or
misaligned shoes; webs glazed or
greasy
linings; and incorrectly
ground or wrong linings, are a few of
the causes for uneven, noisy, pulling.
grabbing, or hard brakes. Adjust or
replace the parts as needed to elimi-
nate the trouble. Lining glaze can
be removed by rubbing the lining
with medium-grade sandpaper until
the lining has a dull finish. Always
adjust the brakes after correcting
any of these brake troubles.
hydraulic system with clean
dena-
tured alcohol before adding n e w
brake fluid.
If the trouble is in the master cyl-
inder, remove and rebuild the cyl-
inder.
If the truck must be moved when
the brakes are locked, open a brake
cylinder bleeder screw for a m o m e n t
to let out a few drops of brake fluid.
This
operation
will
release the
brakes but will not eliminate the
cause of the trouble.
CONTINUED ON NEXT PAGE
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