Pontiac FIREBIRD 1972 Service Manual page 438

5A-2 1972 PONTIAC SERVICE MANUAL
The same types of brake trouble are encountered with
power brakes as with standard brakes. Before checking the
power brake system for the source of trouble, refer to the
trouble diagnosis of standard brakes in Section 5. After
these possible causes have been eliminated, check for the
cause as outlined below:
HARD PEDAL
Vacuum failure due to:
a. Faulty vacuum check valve.
b. Collapsed vacuum hose to manifold.
c. Internal leaks in power brake unit.
Tight pedal linkage.
Power brake unit trouble.
a. Jammed air valve.
b. Vacuum leaks in unit caused by: faulty air valve seal
or support plate seal, damaged floating control valve,
faulty seal of master cylinder, or power cylinder
mounting studs in housings, faulty seal on master
cylinder push rod or a faulty seal of the diaphragm
bead between the housings, or at power piston. It is
also possible to have faulty vacuum check valve
grommet.
c. Defective rolling diaphragm.
d. Restricted air filter elements.
e. Worn or badly-distorted reaction plate or levers.
f. Cracked or broken power piston or reaction retainer.
GRABBY BRAKES (APPARENT OFF-AND-ON
CONDITION)
Power brake unit valve trouble.
a. Sticking air valve.
b.
Restricted diaphragm passage.
Reaction system.
a. Dislodged reaction levers.
b. Broken air valve spring.
c. Worn or distorted levers or plates.
PEDAL GOES TO THE FLOOR OR ALMOST TO
THE FLOOR
Fluid reservoir needs replenishing.
Power brake hydraulic system leakage.
a. Defective primary or secondary cups.
b.
Cracked master cylinder casting.
c. Leaks at wheel cylinder, pipes hoses or connections.
Faulty master cylinder check valve has permitted air to
enter system, causing spongy pedal.
BRAKES FAIL TO RELEASE
Faulty hydraulic check valve.
Blocked passage in power piston.
Air valve sticking.
Broken piston return spring.
Broken air valve spring.
Tight brake pedal clevis pin.
GENERAL DESCRIPTION
The single diaphragm Moraine power brake (Fig. 5A- 1) is
used on A,
F,
G and some X Series vehicles with either
drum or disc brakes. It can be identified by the gold color
of the power unit and master cylinder cover. Always make
sure that the correct master cylinder is used whenever
replacement is necessary.
It is a combined vacuum and hydraulic unit which utilizes
engine intake-manifold vacuum and atmospheric pressure
to provide power assisted application of vehicle brakes.
The unit consists of a power unit and a master cylinder.
From the master cylinder connection outward to the
wheel units, there is no other change in the brake system.
In addition to the master cylinder connections, the unit
requires a vacuum connection to the engine intake-
manifold (through a vacuum check valve) and a mechani-
cal connection to the brake pedal. The unit is
self-contained with no external rods or levers exposed to
dirt or moisture.
The power brake provides lighter pedal pressures and re-
duced pedal travel, making it possible to bring the brake