Air Power Cylinder - GMC 4000 Series Service Manual

bubbles indicates a leaking control valve poppet
air inlet seal.
4. Make a brake application and hold foot p res­
sure on pedal, and observe action of air pressure
gauge at rear of power unit. Power unit should hold
maximum pressure registered on gauge without
perceptible loss until the brake pedal is released.
Loss of air pressure indicates a leaking control
valve poppet exhaust seal, or leakage past the
power cylinder piston.
5. Depress and momentarily hold brake pedal
to several positions between fully released and
fully applied positions. P ressures registered on
gauge at rear of power unit should vary according
to degree brake pedal is depressed.
6. Make a full brake application, then observe
action of air pressure gauge when brakes are re­
leased. If gauge does not return to zero or is slow
in returning, a sticking control valve hydraulic
piston is indicated.
HYDRAULIC PRESSURE TEST
1. Connect a hydraulic pressure gauge capable
of registering at least 1200 pounds pressure to
bleeder valve opening at one of the wheel cylinders.
Do NOT remove air pressure gauge installed at
rear of power cylinder.
2. Apply brakes until approximately 60 pounds
is registered on air gauge at rear of power cylin­
der. Observe reading on hydraulic pressure gauge.
P ressu re should be 950 to 1100 pounds with 60
pounds air pressure applied to power cylinder. If
air system pressure is higher or lower than 60
pounds, hydraulic pressure will be proportionately
higher or lower. Hold brakes applied for at least
one minute, observing action of pressure gauge.
3. A low pressure reading or a drop in hy­
draulic pressure indicates leakage in hydraulic
lines, wheel cylinders, or power cylinder. Inspect
hydraulic lines and wheel cylinders; repair or re­
place damaged parts.
4. After repairs have been made (if neces­
TYPE "F" FRONT BRAKE
(Refer to Figure 26)
Two identical brake shoes are arranged on
backing plate so that their toes are diagonally op-
posite. Two single-end wheel cylinders are ar-
ranged so that each cylinder is mounted between
the toe of one shoe and the heel of the other. The
two wheel cylinder pistons apply an equal amount
of force to the toe of each shoe. Each cylinder
casting is shaped to provide an anchor block for
the brake shoe heel.
Each shoe is adjusted by means of an eccen-
G M C S E R V I C E M A N U A L
sary), repeat test described in paragraph 2 above.
If pressure is still too low or if pressure drops,
an internal leak in the power unit slave cylinder
is indicated. This necessitates removing and over­
hauling the power cylinder.
REMOVAL (Refer to Fig. 25)
as possible to prevent contamination.
fluid which drains from system when hydraulic
lines are disconnected. Disconnect lines. DO.NOT
RE-USE THIS FLUID.
inder to mounting brackets and remove cylinder
from vehicle.
INSTALLATION
fasten to mounting brackets with bolts.
necessary.
check operation of brakes.
hydraulic brakes are part of the air compressor
system and are covered in LUBRICATION (SEC.0)
of this manual.
BRAKE SHOES
trie cam which contacts a pin pressed into brake
shoe web. Each cam is attached to the backing
plate by a cam and shoe guide stud which protrudes
through a slot in the shoe web and in conjunction
with flat washers and C-washers, also serves as
a shoe hold-down. Two return springs are connec-
ted between the shoes, one at each toe and heel,
forward acting (primary shoes), self-energizing
in forward direction of drum rotation. With ve-
h id e in reverse, both shoes are reverse acting
since neither is self-energized in the reverse
HYDRAULIC BRAKES

AIR POWER CYLINDER

REPLACEMENT
1. Tilt cab forward.
2. Clean away as much road dirt and grease
3. Having suitable containers, catch hydraulic
4. Disconnect air lines.
5. Remove bolts which attach air power cyl­
1. Place air power cylinder in position and
2. Connect air lines.
3. Connect hydraulic lines.
4. Check master cylinder and add fluid if
5. Bleed hydraulic system as necessary.
6. Start engine, build up air pressure and
AIR POWER CYLINDER
AIR CLEANER
Air cleaners used with air assisted power
With vehicle moving forward, both shoes are
direction of drum rotation.
Sec. 5A
Page 187
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