Pontiac FIREBIRD 1972 Service Manual page 479

5D-2 1972 PONTIAC SERVICE MANUAL
HARD PEDAL
1. Broken or damaged front or rear hydraulic brake
line.
2. Vacuum failure due to:
a. Faulty vaccuum check valve.
b. Collapsed or damaged vacuum hose to manifold.
c. Plugged or loose vacuum fittings.
3. Bound up pedal mechanism.
4. Power brake unit trouble:
Jammed air valve.
Vacuum leaks in unit caused by faulty air valve
"0" ring seal or support plate seals. Also, a da-
maged, floating control valve, bad seal of master
cylinder, or power cylinder mounting studs in
housings, or a bad seal of the secondary dia-
phragm bead between the housings, or at power
piston. It is possible to have faulty vacuum check
valve grommet.
Defective primary or secondary diaphragm.
Restricted air filter element.
Worn or badly-distorted reaction disc.
Cracked or broken primary or secondary power
piston or piston rod retainer.
Incorrect selective reaction piston.
GRABBY BRAKES
(Apparent Off-and-on Condition)
1. Power brake unit valve trouble:
a. Sticking air valve.
b. Restricted diaphragm passage.
2. Reaction system:
a. Broken air valve spring.
b. Worn or distorted reaction disc or reaction pis-
ton.
PEDAL GOES EITHER TO THE FLOOR OR DROPS
OFF ABOUT HALF THE DISTANCE FROM ITS
NORMAL POSITION
1. Broken or damaged front or rear hydraulic brake
line.
2. Fluid reservoir needs replenishing.
3. Power brake hydraulic leakage:
a. Defective primary or secondary seals.
b. Cracked master cylinder casting.
c. Leaks at front disc brake calipers or rear wheel
cylinders in pipes or connections.
4. Air in hydraulic system.
BRAKES FAIL TO RELEASE
1. Blocked passage in primary or secondary power pis-
ton.
2. Air valve sticking shut.
3. Broken piston return spring.
4. Broken air valve spring.
5. Tight pedal linkage.
GENERAL DESCRIPTION
The tandem diaphragm power brake
-
Moraine is used on
some B Series models. 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.
The Delco Moraine tandem power brake unit is composed
of two major assemblies: The vacuum tandem power head
and the hydraulic dual master cylinder. The dual master
cylinder is similar to those used for standard Delco Mo-
raine power brakes except for an increase in reservoir
capacity.
The power cylinder contains the power piston assembly
and the power piston return spring (1). The power piston
assembly is composed of three basic parts: a plastic pri-
mary power piston (4), a plastic secondary power piston
(6), and a steel housing divider (5).
The primary power piston contains the valving mech-