Pontiac FIREBIRD 1972 Service Manual page 452

SINGLE DIAPHRAGM POWER BRAKE-BENDIX 5B-5
VAI
DIAPHRAGM RETURN
SPRl N G
HYDRAULIC PUSH ROD
-
u
REACTION DISC
CONSTANT VACUUM
'
CHAMBER (FULL VACUUM)
VACUUM PORT (CLOSED)
ATMOSPHERIC PORT
ATMO
PRESS1
SPH
JRE
,
-
TLVE
ROD & PLUNGER
RETURN SPRING
VALVE POPPET VALVE
ER POPPET RETURN SPRING
-
Fig. 58-3 Applying Position
PRINCIPLES OF OPERATION
RELEASED POSITION (Fig. 58-21
With the engine running and the brake pedal released,
vacuum created by the engine removes the air from the
power brake chamber via a hose connection from the
vacuum check valve mounted in the. front housing to a
hose nipple in the base of the carburetor. In the released
position, the valve operating rod and valve plunger are
held to the right (as illustrated) by the valve return spring
thereby closing the atmospheric port and opening the
vacuum port. With the vacuum port open, the chamber to
the rear (right) of the diaphragm is open to vacuum and
the diaphragm is balanced or suspended by vacuum on
both sides. With the power piston assembly in the released
position, the hydraulic push rod and master cylinder pis-
tons are also released and fluid in the two master cylinder
sections is free to return from the brake svstems-to the
fluid reservoirs or to enter either brake system from its
reservoir in order to compensate for the expansion or loss
of fluid in either system.
APPLYING POSITION (Fig. 5B-3)
As the brakes are applied by the driver, the valve operat-
ing rod and plunger assembly moves forward (toward the
left in Fig. 5B-3), in the power piston assembly to com-
press the valve return spring and bring the poppet valve
into contact with the vacuum port seat in the valve hous-
ing to CLOSE the vacuum port. Any additional move-
ment 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 atmospheric pres-
sure through the air filter and passages to the control
vacuum chamber, to the right of the power piston and
diaphragm assembly. With constant vacuum to the left of
the piston and diaphragm, the atmospheric pressure on
the right side exerts force to move the power piston and
.diaphragm and hydraulic push rod and pistons to the left
to close the compensating ports in both systems and force
hydraulic fluid under pressure through the brake tubes
into the brake wheel cylinders. As hydraulic pressure is
developed in the master cylinder, a counter-force (to the
right), acting through the hydraulic push rod and rubber
reaction disc, sets up a reaction force against the vacuum