2-2
GROUP
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 far
DIAGNOSIS GUIDE—STANDARD
BRAKES DO NOT APPLY
EXCESSIVE PEDAL TRAVEL
UNEVEN, NOISY,
GRABBING, OR HARD
OPERATING BRAKES
BRAKES DO 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
mph
to
see if 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
if
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
if 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
if
necessary.
Remove 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.
Remove
the
dirt with
a
scraper
and
an
air
hose.
Drums 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
if
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
it 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
may
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
new
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
moment
to let out
a few
drops
of
brake fluid.
This operation will release
the
brakes
but
will
not
eliminate
the
cause
of the
trouble.
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