Pontiac FIREBIRD 1972 Service Manual page 451

5B-4 1972 PONTIAC SERVICE MANUAL
assembly. This assembly houses the control valve, reaction
mechanism and the power diaphragm return spring. A rod
contacts the rear piston in the hydraulic master clyinder.
The control valve is made up of a single poppet with an
atmospheric port and a vacuum port. A power diaphragm
and return spring give quick response, lighter pedal effort
and improved control.
The reaction mechanism consists of a hard rubber disc
which distributes pressure between the diaphragm plate
and the valve plunger in proportion to their contact areas,
which provides the normal brake feel. A valve operating
rod, which operates the control valve, projects out the end
of the power cylinder housing through a boot and attaches
to the brake pedal. A vacuum seal between the cylinder
housing and the control valve hub seals the rear power
cylinder chamber against atmospheric pressure leaks. The
dual system (double piston) master cylinder attaches to
the vacuum power cylinder front housing. A vacuum seal
between these two units seals the front power cylinder
chamber against atmospheric pressure leaks. A secondary
seal around the rear master cylinder piston prevents hy-
draulic fluid in the master cylinder from leaking into the
cavity of the power cylinder and the front vacuum seal.
Free breathing for this cavity, during braking, is provided
by the two intersecting air passages in the mounting flange
of the master cylinder.
Two hydraulic fluid reservoirs are cast integrally with the
master cylinder. A rubber diaphragm covers both reser-
voirs to seal the brake system from contamination and
prevent corrosion and subsequent leakage. Inside the mas-
ter cylinder, there are parts for the two separate hydraulic
systems: a rear piston assembly, which includes a second-
ary seal, primary cup and spring, screw and spacer; a front
piston assembly, which includes two back-to-back second-
ary seals, a primary cup and a spring and spring retainer
and a piston stop screw and seal.
.
/
PRESSURE
VACUUM PORT (OPEN)
ATMOSPHERIC PORT
DIAPHRAGM PLATE
(POWER PISTON)
PUSH ROD
REACTION DISC
CONSTANT VACUUM
(FULL
VACUUM)
,
LVE
'ERATI
ROD
VALVE ROD
8,
m--
\
PLUNGER RETURN
VALVE VALVE
\
POPPETVALVE
PLUNGER POPPET RETURN SPRING
Fig. 58-2 Released Position