Pontiac FIREBIRD 1972 Service Manual page 481

5D-4 1972 PONTIAC SERVICE MANUAL
screw
(43)
is located in the bottom of the front fluid reser-
voir. The stop screw aids in assembly and prevents rear-
ward movement of the secondary piston when pressure
bleeding.
Connection is made to the front disc brake caliper pistons
and rear wheel cylinders through the two hydraulic out-
lets (46).
PRINCIPLES OF OPERATION
A line from a vacuum source on the engine is connected
to the vacuum check valve in the front housing of the
power brake. The check valve serves to prevent loss of
vacuum in the power brake when the vacuum from the
engine is low or not available:
At rest, the position of the power piston assembly is deter-
mined by the primary piston which is held against the rear
housing. The air valve spring holds the air valve back, so
that it retaining ring rests against the primary power pis-
ton.
In the released position an equal amount of vacuum is
present on both sides of both 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.
With the power head in the released position, the primary
seals on the primary piston and secondary piston are held
back past the two by-pass holes in the bore of the master
cylinder. In this position, no hydraulic pressure is present
in the system.
APPLYING POSITION (FIG. 5D-3)
RELEASED POSITION (FIG. 5D-2)
In the released position the air valve is seated on the
floating control valve, and prevents the floating control
valve from seating on the primary power piston. The float-
ing control valve assembly is held against the air valve seat
by a spring. Atmospheric air is thus shut off to the cham-
bers behind the diaphragms.
As the brake pedal is depressed, the air valve push rod
assembly moves in the direction of the matster cylinder.
The floating control valve, being loaded by the spring
immediately behind it, follows the air valve until it con-
tacts the seats on the primary power piston. With the
floating control valve seated on the primary power piston,
a passage way is closed and the vacuum source is shut off
Fig.
5D-2
Released Position