Pontiac FIREBIRD 1972 Service Manual page 483

5 D - 6
1972 PONTIAC SERVICE MANUAL
Fig. 5D-4 Holding Position
ring rests against the primary power piston. The contact
of the air valve assembly with the floating control valve
shuts off the flow of atmospheric pressure air. As the air
valve spring forces the air valve assembly into its released
position, the air valve lifts the floating control valve from
its seat on the primary power piston. The air valve seating
on the floating control valve shuts off the outside air. The
unseating of the floating control valve from the power
piston opens the vacuum source to the area to the right of
the power pistons.
Vacuum, which is present at all times in the spaces to the
left of each power piston is now also present on the right
side of each power piston.
Now that both sides of both power pistons are exposed to
equal vacuum, a spring will return to the released or at rest
position. This will release the master cylinder.
If the brake pedal is released quickly, the master cylinder
pistons immediately return to the released position. When
the fluid in the lines cannot return as quickly as the pis-
tons, a low pressure area is created in front of both pri-
mary cups. This low pressure area is compensated for by
the flow of fluid from the reservoir through the compen-
sating holes to the space between the primary cups and
secondary seals and through the holes in the pistons. The
excess fluid introduced into the system during compensa-
tion will return to the fluid reservoirs through the small
by-pass holes in the master cylinder bore.
VACUUM SOURCE FAILURE
In case of vacuum source interruption, enough vacuum is
available in the power brake to make about three power
assisted stops.
If the vacuum check valve should fail or if the vacuum
stored in the unit is exhausted, it is still possible to operate
the power brake manually. Considerable effort is required,
however.
CHECKS AND ADJUSTMENTS ON CAR
1. Check for free operation of brake pedal. If binding
exists, check all pivot points for binding and lubricate
as required.
2. Check stoplight switch for proper setting and operat-
ion, see Section 5.
3. Check fluid level in hydraulic cylinder reservoirs.
Fluid level should be as shown in Fig. 5-1 1.
4. Check vacuum line and connections at carburetor
and vacuum check valve for possible vacuum leaks.
5. Check engine for good stall
-
free idle and correct as
required.