Pontiac FIREBIRD 1972 Service Manual page 453

5B-6 1972 PONTIAC SERVICE MANUAL
VACUUM CHECK
VACUUM PORT (CLOSED)
DIAPHRAGM PLATE
ATMOSPHERIC PORT
(POWER PISTON)
DIAPHRAGM RETURN ATMOSPHERIC
SPRING
REACTION DISC
VALVE POPPET RETURN
GER POPPET SPRING
CONSTANT VACUUM CONTROL VACUUM CHAMBER
(PARTIAL VACUUM)
Fig. 5 8 - 4 Holding Position
power piston and valve plunger. The rubber disc distrib-
utes the pressure between the vacuum power piston and
the valve plunger in proportion to their respective contact
areas.
The pressure acting against the valve plunger and valve
operating rod tend to move the valve plunger slightly to
the right in relation to the valve housing of the power
piston to close off the atmospheric port. Since part of the
counter-force (to the right) reacts through the valve
plunger and valve operating rod against the driver's foot,
a feel of the braking effort is provided. This reaction force
is in direct proportion to the hydraulic pressure developed
within the brake system. As shown in Fig. 5B-3, full power
application has been attained with full atmospheric pres-
sure admitted to the control vacuum chamber (to the
right) and with constant vacuum to the left of the power
piston and diaphragm assembly. Any increase in hydrau-
lic pressure to the wheel brakes beyond this point must be
supplied by physical effort of the driver.
HOLDING POSITION (Fig. 58-4)
During application of the brakes, the reaction against the
valve plunger is working against the driver to close the
atmospheric port. With both the atmospheric and vacuum
ports closed, the power brake is in the holding position.
When both ports are closed, any degree of braking ap-
plication attained will be held until either the atmospheric
port is reopened by an increase in pedal pressure to further
increase the brake applicqtion or by a decrease in pedal
pressure to re-open the vacuum port to decrease the brake
application. Whenever the pressure applied to the brake
pedal is held constant for a moment, the valve plunger and
poppet return to their holding position. However, upon
reaching the fully applied position (as shown in Fig. 5B-3),
the valve plunger is held away from the valve poppet seat
to admit atmospheric pressure to the control vacuum
chamber (to the right) of the power piston and diaphragm.
As the power piston and hydraulic master cylinder piston
move back, the fluid from the wheel cylinders flows back