Pontiac FIREBIRD 1972 Service Manual page 465

TANDEM DIAPHRAGM POWER BRAKE-BENDIX 5C-5
assembly. A valve return spring returns the valve plunger
and valve rod to the released position when pressure is
released from the brake pedal. The valve poppet is of the
flexible rubber type and is supported by the valve body.
In the released position, a return spring holds the poppet
against the atmospheric port seat. A special type of seal
is used for the opening between the valve body sleeve and
the rear shell. Vacuum is supplied to the power brake unit
from the base of the carburetor, through a vacuum hose
and check valve. Atmospheric air for operation is admit-
ted through the air filter located inside the rubber dust
guard and through silencers and filters within the valve
housing. A rubber valve rod guard keeps dust and dirt
from getting into the valve mechanism.
At its front end, the unit incorporates another special seal
to close the opening between the front shell and the hy-
draulic push rod. The hydraulic push rod is the link be-
tween the vacuum power diaphragm assembly and the
hydraulic piston of the master cylinder. The center plate
located between the two diaphragms divides the power
unit into four chambers. A long, threaded hub on the front
plate slides through the center hole in the center plate and
is screwed into the hub of the rear plate to maintain a set
separation between the two diaphragms as the front and
rear plates and diaphragms move backward and forward.
This front plate hub is sealed by a vacuum seal that is
permanently installed in the stationary center plate.
PRINCIPLES OF OPERATION
RELEASED POSITION (Fig. 5C-3)
With engine running and the brakes released, vacuum
from the intake manifold is admitted through the vacuum
check valve to the front (left) vacuum chamber and,
a
T O ENGINE V A C U U M
PHERIC PRESSURE
CONTROL V A C U U M CHAMBERS
(FULL VACUUM)
Fig. 5C-3 Released Position