Pontiac FIREBIRD 1972 Service Manual page 462

5C-2 1972 PONTIAC SERVICE MANUAL
3. VACUUM LEAK IN HOLDING POSITION
With transmission in neutral or park, stop engine while
holding a moderately heavy load steadily on pedal. After
one minute release and apply pedal several times. If there
is no vacuum assist during this test but system was normal
during test No. 1 above there is an air leak within unit.
NOTE: Some units on this test will leak air in-
ternally if pedal load is light. This is a normal
cundition.
4. HYDRAULIC LEAK
a. Depress brake pedal while engine is running, main-
taining constant pressure. If pedal falls noticeably in
one minute, the hydraulic system is leaking.
b. If pedal has a spongy feel when applying brakes, air
may be present in the hydraulic system.
Road test brakes by making a brake application at
about 40 mph to determine if vehicle stops evenly and
quickly.
If system checks are satisfactory and the brake pedal
travels to within 1" of floor-board, brake shoes re-
quire adjustment or replacement. Lubricate self-
adjusting components.
3. Solid hub seals should never be installed be-
tween the master cylinder and front housing.
c. Mechanical.
1. Badly dented vacuum cylinder.
2. Bound up pedal linkage.
3. Improperly adjusted stop light switch.
4. Galled valve plunger.
5. Broken or missing springs.
GRABBY BRAKES (APPARENT OFF-AND-ON
CONDITION)
1. Faulty pedal linkage.
2. Dented vacuum cylinder.
3. Sticking valve plunger.
4. Defective vacuum check valve.
5. Loose vacuum connections.
PEDAL GOES TO FLOOR OR ALMOST TO FLOOR
HARD PEDAL
1. Brake adjustment.
1. Vacuum failure due to:
a. Faulty vacuum check valve.
b. Vacuum hose or pipe collapsed, plugged, kinked
or disconnected.
c. Internal leaks in power brake unit.
2. Tight brake pedal clevis pin.
3. Power brake unit trouble.
a. Vacuum Leaks in unit caused by improper assem-
bly, missing parts, damaged parts and/or foreign
material.
b. Hydraulic.
1. Cups swollen by improper fluid.
2. Compensating port not cleared by primary cup.
2. Air in hydraulic system.
3. Fluid reservoir empty.
4. Hydraulic fluid leakage:
a. Defective filler cap, missing bame, or filler cap
gasket.
b. Cracked master cylinder casting.
c. Leaks at wheel cylinder, in pipes, hose or at con-
nections.
d. Defective secondary seal on master cylinder pis-
ton. (Brake fluid in cavity around master cylinder
for front vacuum seal).
e. Faulty primary cup which causes pedal to sink to
the floor under constant load but does not empty
reservoir.