Chevrolet Light Duty Truck 1973, Service Manual

Page: 268 / 979

5-2 BRAKES
GENERAL DESCRIPTION
All vehicles are equipped with a dual hydraulic brake
system.
The split system consists basically of two separate brake
systems. When a failure is encountered on either, the
other is adequate to stop the vehicle. If one system is not
functioning, it is normal for the brake pedal lash and
pedal effort to substantially increase. This occurs because
of the design of the master cylinder which incorporates
an actuating piston for each system. When the rear
system loses fluid, its piston will bottom against the front
piston. When the front system loses fluid, its piston will
bottom on the end of the master cylinder body. The
pressure differential in one of the systems causes an
uneven hydraulic pressure balance between the front and
rear systems. The combination valve, near the master
cylinder, (except RPO H22 vehicles) detects the loss of
pressure and illuminates the brake alarm indicator light
on the instrument panel. The pressure loss is felt at the
brake pedal by an apparent lack of brakes for most of
the brake travel and then, when failed chamber is
bottomed, the pedal will harden.
RPO H22 vehicles (with frame mounted vacuum over
hydraulic boosters) have an electrical switch that senses
pedal travel. This switch will illuminate the lamp on the
instrument panel whenever the brake pedal travel is in
excess of 5.10 inches.
If a vehicle displays these symptoms, it is a good
indication that one of the systems contains air or has
failed, and it is necessary to bleed or repair the brakes.
MASTER CYLINDER (Fig. 1)
The system is designed with a separate hydraulic system
for the front and rear brakes using a dual master
cylinder. The cylinder has two separate reservoirs and
outlets in a common body casting. On all 10 and G20
series vehicles, the front reservoir controls the front
brake system and the rear reservoir controls the rear
system. On all other 20-30 series vehicles, the front
reservoir controls the rear brake system and the rear
reservoir controls the front system (except on CA 30 with
vacuum over hydraulic boosters where the front is
controlled by the front booster and rear by the rear
booster).
COMBINATION VALVE
All models (except models with vacuum over hydraulic
boosters) have a combination valve. The combination
valve is located below the master cylinder on P and G
models; on the front crossmember on C-K models. The
front and rear hydraulic lines are routed through this
combination "metering" and "brake failure warning
switch" to their appropriate wheel cylinders or caliper.
The metering portion of the combination valve tends to
"hold off" front hydraulic pressure until the rear brake
system overcomes their pull back springs; then pressure
is allowed to flow with the result being a good
distribution of braking effort.
The brake failure warning switch portion of the
combination valve "senses" a loss of hydraulic pressure,
if a failure should occur and turns "on" a red light in
the dash to warn the operator of the failure.
DISC BRAKES FRONT—(Fig. 2)
All models have disc brakes on the front. The one piece
caliper mounts on the steering knuckle/steering arm,
which is also a one piece casting, and astride the brake
disc. The caliper is the single piston design which is said
to be a sliding caliper sliding piston. No front brake
adjustment is necessary once the system is in operation
and the pedal has been stroked to "seat" the shoes to the
caliper.
DRUM BRAKES REAR—(Fig. 3)
The rear brakes are duo servo and self adjusting (except
R.P.O. H-22). Brake adjustment takes place when the
brakes are applied with a firm pedal effort while the
vehicle is backing up. Applying the brakes moves the
actuator which turns the star wheel and lengthens the
adjuster screw assembly. This action moves the shoes
outward until clearance between the lining and drum is
within proper limits.
LIGHT DUTY TRUCK SERVICE MANUAL