Pontiac FIREBIRD 1972 Service Manual page 466

5C-6 1972 PONTIAC SERVICE MANUAL
through a port in the hub of the front plate, to the vacuum system to the fluid reservoir or enter the brake system
chamber to the front (left) of the rear diaphragm. In the
from the reservoir to compensate for expansion or loss of
released position (no pressure applied to the brake pedal),
fluid from the brake system.
the valve operating rod and valve plunger are held to the
right in the valve housing by the valve return spring to
close the atmospheric port and open the vacuum port.
With the valve in this position, the chambers to the rear
of both the front and rear diaphragm are open to vacuum
through the portings in the hub of the valve housing and
around the edge of the center plate. The vacuum power
diaphragms are then balanced or suspended in vacuum,
since vacuum is present on both sides of both diaphragms.
The vacuum power diaphragm return spring is then free
to return the diaphragm and plate assembly with the hy-
draulic push rod to the fully released position.
With the hydraulic push rod in the released position, the
hydraulic compensating port in the master cylinder is
open to permit brake fluid either to return from the brake
APPLIED POSITION (Fig. 5C-4)
As the brakes are applied by the driver, the valve operat-
ing rod and valve plunger move to the left in the dia-
phragm and plate assembly to compress the valve return
spring and bring the poppet valve into contact with the
vacuum valve seat in the vale housing to close the vacuum
port. Any additional movement of the valve operating rod
in the applied direction moves the valve plunger away
from the poppet valve to open the atmospheric port and
admit atmosphere through the air filter, poppet and pas-
sages to the pressure chambers at the right sides of both
the front and rear diaphragms.
JRE
iRS
Fig. 5C-4 Applying Position