Pontiac FIREBIRD 1972 Service Manual page 482

TANDEM DIAPHRAGM POWER BRAKE-MORAINE 5D-5
Fig. 5D-3 Applied Position
from the areas behind both power pistons. As the air valve
continues to move in the direction of the master cylinder
it unseats from the floating control valve.
When the air valve unseats from the floating control valve,
a passage is opened to air at atmospheric pressure. Air at
atmospheric pressure travels through holes in the boot,
around the silencer, through the filter, through the center
of the floating control valve and through the open passage
into the areas behind both power pistons.
In this position a differential atmospheric pressure forces
the power pistons to travel toward the master cylinder.
As the power pistons travel in the direction of the master
cylinder, the piston rod actuates the master cylinder.
As pressure builds up ahead of the master cylinder pri-
mary piston, an equal force is also created in the direction
of the brake pedal. This reaction force on the end of the
piston rod is transmitted through the piston rod retainer.
The piston rod retainer compresses the rubber reaction
disc and causes the reaction disc to exert a portion of the
reaction force against the reaction piston. This results in
a movement of the reaction piston toward the air valve.
The air valve and the reaction piston contact each other
to transmit this reaction force through the push rod to the
brake pedal.
HOLDING POSITION (FIG. 5D-4)
When the desired braking is reached and no additional
pressure is applied to the brake pedal, the power brake will
go into the holding or poised position.
The power pistons travel toward the master cylinder only
far enough to bring the floating control valve into contact
with the air valve assembly shutting off the flow of air.
Since the floating control valve has maintained its seated
position on the power piston, the vacuum source is still
shut off to the areas behind the power pistons.
In the hold position, the rubber reaction disc is com-
pressed and extrudes into the cavity in the area of the
reaction piston. The reaction piston is now in full contact
with the end of the air valve.
The reaction force and the apply force at the push rod are
now balanced; therelative positions of the power pistons,
valving, the reaction mechanism will be maintained until
either more pressure is applied to the brake pedal or the
brake pedal is released.
RELEASING POSITION (FIG. 5D-5)
As the pressure at the brake pedal is released, the air valve
spring forces the air valve assembly back until its retaining