Chevrolet 1977 light duty truck Service Manual page 386

brake travel and then, w hen failed ch am b er is bottomed,
the pedal will harden.
If a vehicle displays these symptoms, it is a good
indication that one o f the systems contains air or has
failed, an d it is necessary to bleed or repair the brakes.
M A S TE R CYLINDER
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 co m m o n body casting.
C O M B IN A T IO N VALVE
All models have a com bination valve. The front and
rear hydraulic lines are routed through this com bination
" m e te rin g " and "b ra k e failure w arning switch" to their
a pp rop riate wheel cylinders or caliper.
The m etering portion of the com bination valve
tends to "hold off" front hydraulic pressure until the rear
brake system overcomes the pull back springs; then
pressure is allowed to flow with the result being a good
distribution of braking effort.
T he brake failure w arning switch portion o f the
c o m bination valve "senses" a loss o f hydraulic pressure,
IN S P E C T IO N AND TE S TIN G BRAKES
Testing Brakes
CAUTION: New linings m ust be protected fro m
severe use f o r several hundred miles.
Brakes should be tested on dry, clean, reasonably
smooth and level roadway.
perform an ce cann ot be m ade if roadway is wet, greasy
o r covered with loose dirt so that all tires do not grip the
road equally. Testing will also be adversely affected if
roadw ay is crowned so as to throw weight o f vehicle
toward wheels on one side or if roadway is so rough that
wheels tend to bounce.
Test brakes at different vehicle speeds with both
light and heavy pedal pressure; however, avoid locking
the wheels and sliding the tires on roadway. Locked
wheels and sliding tires do not indicate brake efficiency
since heavily braked but turning wheels will stop vehicle
DIAGNOSIS
A true test o f brake
if a failure should occur, and turns " o n " a red light in
the dash to w arn the operator of the failure.
DISC BRAKES FRO NT
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, an d astride the
brake disc. The caliper is the single piston design which
is said to be a sliding caliper sliding piston. N o front
brake adjustment is necessary once the system is in
operation and the pedal has been stroked to "s ea t" the
shoes to the caliper.
DISC BRAKES REAR (JF9)
R ear disc brakes operate in the same m an ner as
front disc brakes except the caliper is mounted to a
support attached to the axle flange.
D RU M BRAKES REAR
The rear brakes are duo servo and self adjusting.
Brake adjustment takes place w hen 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.
in less distance than locked wheels. More tire-to-road
friction is present with a heavily braked turning tire than
with a sliding tire.
External Conditions that Affect Brake Performance
1. Tires--Tires having unequal contact and grip on
road will cause unequal braking. Tires must be equally
inflated and tread p attern o f right and left tires must be
approximately equal.
2. Vehicle Loading--When
loading, the most heavily loaded wheels require more
braking power than others. A heavily loaded vehicle
requires more braking effort.
3. Front Wheel Bearings-A
bearing permits the drum to tilt and have spotty contact
with the brake shoe linings causing erratic action.
4. Front End A lignment-M isalignm ent o f the front
end, particularly in regard to limits on cam ber and
theoretical king pin inclination, will cause the brakes to
pull to one side.
vehicle has unequal
loose front wheel