Buick LeSabre 1965 Service Manual page 548

9 - 2 6 POWER BRAKES
BRAKES
VACUUM
CHAMBER
RUBBER DIAPHRAGM
DIAPHRAGM
SUPPORT PLATE
VACUUM CHAMBER
BOOT
AIR FILTERS
MASTER CYLINDER
PISTON
MASTER
CYLINDER PUSH ROD
POWER PISTON
RETURN SPRING
PUSH ROD
FLOATING
CONTROL VALVE
AIR VALVE
REACTION
PLATE
REACTION
LEVERS
Figure 9-16—Power Brake Unit - Released Position
This air is drawn through two
small passages in the power pis-
ton, over the valve seat in the
power piston and then through
the power piston into the space at
the left of the power piston. It is
then drawn through the check
valve and thence to the vacuum
source.
In this position there is vacuum
on both sides of the power piston,
and the power piston is held
against the rear housing by the
power piston return spring. At
rest, the hydraulic reaction plate
is held against the reaction re-
tainer. The reaction levers are
leld back against the hydraulic
eaction plate by the air valve
pring. The air valve spring holds
the air valve back so that its
retaining ring rests against the
power piston.
The floating control valve as-
sembly is held against the air
valve seat by the floating control
valve spring. In this position, the
by-pass hole in the hydraulic
master cylinder is open to the
reservoir and fluid can flow
freely in either direction between
the hydraulic cylinder and the
fluid reservoir.
A residual pressure is maintained
in the brake lines by the check
valve and its spring.
(2) Applying Position
As the pedal is depressed, the
push rod carries the air valve
away from the floating control
v a l v e . See F i g u r e 9-17. The
floating control valve will follow
until it is in contact with the
raised seat in the power piston.
When this occurs, the vacuum is
shut off to the right hand side of
the power piston, and air under
atmospheric pressure r u s h e s
through the air filter and travels
past the seat of the air valve and
through two passageways into the
housing on the right of the power
piston.
Since there is still vacuum on the
left side of the power piston, the
force of the air at atmospheric
pressure on the right of the pis-
ton will force the power piston to
travel to the left.