GMC 4000 Series Service Manual page 193

Sec. 5A
Page 186
HYDRAULIC BRAKES
M a s t e r C y lin d e r
Line To
C ontrol V a lv e
Line To
W h e e l C y lin d e r
A ir T a n k To
C ontrol V a lv e
I
Li
To W h e e l
C y lin d e rs
To C ontrol
V a lv e
P o w e r C y lin d e r!
Figure 2 5 — Typical Installation for Air-Assisted
Hydraulic Brakes on Tilt Cab M odels
a piston, and a push rod that connects the power
piston to the hydraulic piston.
2. A hydraulic cylinder which includes a hy­
draulic piston with built-in check valve and a re­
sidual pressure check valve.
3. An air pressure control valve which con­
trols the power output of the air pressure cylinder
in accordance with the hydraulic pressure develop­
ed within the brake master cylinder.
The compressed air supply line is attached to
the control valve. Air is exhausted from the ex­
haust port in the control valve through an exhaust
G M C S E R V I C E M A N U A L
kJf ..,
To Trailer
Line B ra k e s
Tank
A ir
tube directed downward to prevent the entrance of
dirt or water into the power cylinder.
The hydraulic input line from the master cyl­
inder is attached to the fluid inlet port located in
the control valve. The outlet port, located in the
end cap, is connected to the wheel cylinders of the
vehicle brakes.
When the brake pedal is depressed, hydraulic
fluid is transmitted from the master cylinder to
the control valve. /The force on the control valve
piston moves the valve diaphragm assembly, which
closes the atmospheric port and opens the com­
pressed air port. The compressed air admitted to
the air cylinder moves the air piston and push rod
to displace fluid to the wheel cylinders and apply
the brakes.
When pressure on brake pedal is removed, the
force of the brake shoe return springs, the air
piston return spring and the brake pedal return
spring cause movement in the opposite direction
of all components and the brakes are released.
Opening of the atmospheric port in the control
valve allows compressed air to vent to atmosphere.
LUBRICATION
Refer to LUBRICATION (SEC. 0) for type of
lubricant, recommended periods of application,
and method of applying.
AIR POWER BRAKE SYSTEM TESTS
OPERATING TEST
1. Build up air pressure in system to normal
operating pressure. If vehicle is equipped with
trailer brake controls, trailer line shut-off cocks
must be closed while making tests.
2. Apply brakes, then listen for sound of ex­
hausting air pressure as brakes are released.
Rapid release of air pressure indicates that power
unit is operating.
3. Depress brake pedal and hold foot pressure
on pedal. If pedal gradually falls away, leakage in
hydraulic system is indicated. Make further test
as directed under "Hydraulic Pressure T est."
AIR PRESSURE TESTS
1. Remove lubrication pipe plug from rear end
of cylinder shell and connect an air pressure test
gauge at this point. Build up air pressure in system
to maximum limit.
2. Coat all air line connections with soap suds
to check for leakage. Leakage can som etim es be
corrected by tightening the connection. If this fails
to correct leakage, new fittings, metal tubing, or
flexible hose must be used.
3. Hold a jar of water up under exhaust tube
so that end of tube is immersed in water. Watch
for bubbles to appear in water. The appearance of