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P A R T 2-1-GENERAL BRAKE SERVICE
25
draulic pressure will be proportion-
ately higher or lower than the pres-
sure given in Part 2-4.
Hold
the brakes in the fully-
BRAKE PEDAL ADJUSTMENT
W h e n the brake pedal free-travel,
which is the movement of the brake
pedal before the push rod touches
the master cylinder piston, is less
than
3/
i8 inch or more than
3
/s inch
(Fig. 2), the pedal should be ad-
justed.
!. Push the brake pedal d o w n by
hand pressure, and check the free
travel.
2. Loosen the lock nut on the ec-
centric bolt, and rotate the eccentric
bolt until the free travel is within
3
/ia-% inch.
O n a P-Series truck, turn the hex
head of the push rod to obtain the
required free-travel.
3. Hold the bolt securely, and
torque the lock nut to 30-35 ft-lbs.
4. Recheck the pedal free-travel
to m a k e sure that the adjustment did
not change w h e n the lock nut was
tightened.
HYDRAULIC SYSTEM BLEEDING
When any part of the hydraulic
system (Fig. 1) has been discon-
nected for repair or replacement, air
m a y get into the lines and cause
spongy pedal action. This requires
the bleeding of the hydraulic system
after i t has been properly connected,
to be sure all air is expelled from the
brake cylinders and lines. The hy-
BRAKE PEDAL FREE TRAVEL
HI Oil-A
FIG. 2—Pedal Free Travel Check
applied position for at least one min-
ute, and note the reading on the
pressure gauge. T h e hydraulic sys-
tem should hold pressure for at least
draulic system can be bled manually
or with pressure bleeding equipment.
'
W h e n bleeding the brake system,
Meed one brake cylinder at a time
beginning at the cylinder with the
longest hydraulic line. If the brake
assembly is equipped with two cyl-
inders, always bleed the upper cyl-
inder first. Keep the master cylinder
y
reservoir filled with new heavy-duty
e
brake fluid during the bleeding op-
eration. Never use brake fluid which
has been drained from the hydrau-
lic system.
n
If the hydraulic system is equipped
with a vacuum booster, bleed the
x
hydraulic section of the booster be-
e
fore bleeding the rest of the hydrau-
lic system. T h e bleeding operation
must be done with the engine off and
with no vacuum in the system.
3.
T o bleed the hydraulic section of
;
'
a vacuum booster, follow steps 1
d
through 4 of the manual bleeding
IS
procedure, attaching the drain tube
to the bleeder screw at the end plate
of the booster (or the bleeder screw
}
nearest the power chamber). Repeat
this procedure at the other bleeder
lc
screw i f the booster is so equipped.
1 -
ir
M A N U A L BLEEDING
1. Attach a rubber drain tube to
the bleeder screw of the brake wheel
,
cylinder. T h e end of the tube should
f i t snugly around the bleeder screw.
/-
2. Submerge the free end of the
tube in a container partially filled
with clean brake fluid. Loosen the
bleeder screw.
3. Push the brake pedal d o w n
slowly by hand, close the bleeder
screw, then allow the pedal to return
slowly to the fully-released position.
Repeat this operation until air bub-
bles cease to appear at the submerged
i
end of the tube.
4. W h e n the fluid is completely
free of air bubbles, close the bleeder
screw and remove the drain tube.
5. Repeat this procedure at each
brake cylinder. Refill the master cyl-
A
inder reservoir after each brake cyl-
inder is bled and when the bleeding
operation i s completed.
one minute without losing pressure.
A low pressure reading or a drop in
pressure, indicates leakage in the
booster or in the hydraulic system.
P R E S S U R E
B L E E D I N G
Be sure that the tank is clean, that
there is enough new heavy-duty brake
fluid in the bleeder tank to complete
the bleeding operation and that the
tank is charged with 10-30 pounds of
air pressure.
1. Clean all dirt from around the
filler hole on the top of the master
cylinder reservoir, and attach the
bleeder tank hose to the filler hole.
2. Attach a rubber drain tube to
the bleeder screw of the brake cyl-
inder. The end of the tube should fit
snugly around the bleed screw.
3. Submerge the free end of the
tube in a container partially filled
with clean brake fluid, and then loos-
en the bleeder screw.
4. Open the valve on the bleeder
tank to admit pressurized brake fluid
to the master cylinder reservoir (or
line).
5. W h e n air bubbles cease to ap-
pear in the fluid at the submerged
end of the drain tube, close the bleed-
er screw and remove the tube.
6. Repeat this procedure at each
brake cylinder.
7. W h e n the bleeding operation is
completed, close the bleeder tank
valve and remove the tank hose from
the filler hole.
8. Refill the master cylinder reser-
voir to within
3
/» inch from the top
of the filler neck.
WHEEL CYLINDER
REPLACEMENT OR REPAIR
REPLACEMENT
1. Remove the wheel, drum, and
brake shoes. Remove the cylinder-to-
shoe connecting links.
2. Disconnect the brake line from
the brake cylinder. O n mucks equip-
ped with a vacuum or air booster, be
sure the engine is stopped and there
is no vacuum or air pressure in the
system before disconnecting the hy-
draulic lines.
3. R e m o v e the brake cylinder re-
taining bolts and lockwashers, and
then remove the cylinder from the
carrier plate. O n the two-cylinder
| COMMON ADJUSTMENTS AND REPAIRS
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