Chevrolet Light Duty Truck 1973, Service Manual

Page: 315 / 979

BRAKES 5 -4 9
PRELIMINARY CHECKS
External Conditions That Affect Performance
In addition to previously mentioned conditions, the
following external conditions may affect brake perform­
ance and should be corrected before work is done on the
brake mechanism.
Tires
Tires having unequal contact and grip on the road could
cause unequal braking. Tires should be equally inflated
and tread pattern of right and left tires should be
approximately equal.
Loading
When the vehicle has unequal loading, the most heavily loaded
wheels require more braking power than others.
Shock Absorbers
Faulty shock absorbers that permit bouncing of the vehicle on
quick stops may give the erroneous impression that brakes are
too severe.
General Checks
NOTE: If a damaged component or malfunc­
tion is discovered while making any of the
following brake system checks, repairs must
be made before attempting to continue with
additional diagnosis.
1. Inspect for excessive tire tread wear and indications
of front suspension misalignment. Check tire
pressures and for improperly adjusted or worn
wheel bearings. Any of these conditions can result
in an improper diagnosis.
2. Check master cylinder fluid level. If low, refill to the
proper level, pressurize the system, and make an
inspection of the entire brake system for leakage.
Wheel cylinder leakage can frequently be detected
by the presence of brake fluid on the inboard side
of the wheel and tire.
3. Power Brake:
a. With transmission in Park, stop the engine, and
exhaust all vacuum in the system by depressing
the pedal several times.
b. Depress the brake pedal and hold it in the apply
position for one minute with approximately
20
lbs. pedal force. If the pedal gradually falls
away or the brake warning light comes on, it is
an indication that the hydraulic system is
leaking or that there is a malfunction in the
master cylinder. Check all tubing, hoses, wheel
cylinders, calipers and connections for leakage
before replacing the master cylinder.
c. If the brake pedal feels spongy, it is an
indication of air in the hydraulic system. Bleed
the air from the system and recheck the pedal
feel.
NOTE: Steps (b) and (c) may also be used to
check for leakage or the presence of air in a
non-vacuum powered brake system.
d. Depress the brake pedal and start the engine. If
the vacuum system is operating, the brake pedal
will tend to fall away when the engine starts and
less pedal pressure will be needed to hold it in
the applied position. If no action is felt when
the engine starts, the vacuum system is
inoperative.
ROAD TESTING
CAUTION:
A road test should be made only
when the operator is sure that the brakes will
stop the vehicle.
Road tests are necessary to check brakes for safe, quiet
performance. Preliminary inspection should be made in
the shop as outlined in the preceding paragraphs. The
following tests will aid in the evaluation of brake
performance and the need for service. They should be
conducted on dry, clean, reasonably smooth and level
roadway. Use care not to induce fade unintentionally
with continuous applications during test. Refer to the
Troubleshooting Chart for causes and remedies for
trouble which may be discovered during testing.
WARNING:
Before driving any vehicle, push
the brake pedal to make sure it will not
bottom. Next, make a series of slow speed
stops to determine if the brakes are safe for
driving.
Low Speed Test for Effectiveness, Pulls and
Noise fk
S .
Make light and medium stops at from 10 to 15 MPH.
Bring the vehicle to a complete stop each time. Observe
the effort required to make each stop. Is it too light
(grabby) satisfactory or too hard? Check for pulls.
Unequal front brakes will cause pulls in the direction of
the brake doing the most work. Unequal rear brakes may
not cause noticeable pulling during low speed stops.
Check for noise. Open the windows and turn off all
accessories and listen to determine the type of noise and
the wheel from which it is coming.
NOTE: Driving along a wall when applying
brakes will make noises more audible.
High Speed Test for Roughness or
Pulsations
Make light stops from 60 MPH or maximum legal speed.
Check for roughness or pulsations by pedal feel and
vehicle vibration.
High Speed Test for Effectiveness, Pulls and
Noise
Make hard stops (just short of skid) from 60 MPH. Bring
LIGHT DUTY TRUCK SERVICE MANUAL