Buick LeSabre 1965 Service Manual page 561

BRAKES
POWER BRAKES 9 - 3 9
RUBBER
DIAPHRAGM'
COMPENSATING
PORT
RESERVOIR
CHAMBER DIAPHRAGM
POWER PISTON
REACTION DISC
BOOT
OPERATING
ROD
•SILENCER
FILTER
'AIR VALVE
RETURN SPRING
AIR VALVE
POWER PISTON
RETURN SPRING
Figure 9-35—Power Brake and Master Cylinder Assembly - Cross Section
to seal the opening between valve
body sleeve and the rear end
plate. Vacuum is supplied to the
p o w e r unit through a vacuum
check valve located in the front
shell. Air for operating is ad-
mitted through six holes in the
end of the rubber valve rod boot
and passes through three filters
which protect all internal operat-
ing parts from dust and dirt. A
seal located in the front vacuum
chamber seals the opening be-
tween the vacuum chamber and
the hydraulic push rod. The hy-
draulic p u s h r o d is the link
between the vacuum power dia-
phragm assembly and the piston
of the hydraulic master cylinder.
9-23 OPERATION OF
POWER BRAKE
a. Released Position
With the engine running and the
brakes released, vacuum from the
intake m a n i f o l d i s admitted
through the vacuum check valve
to the front (left) vacuum cham-
ber. In the released position (no
pressure applied to t h e brake
pedal), the valve operating rod
and air 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 chamber
to the rear of the power piston is
also open to vacuum through the
porting in the valve housing. Since
vacuum is present on both sides
of the diaphragm, the power
piston return spring is free to re-
turn the power piston assembly
and the hydraulic push rod to the
fully released position. With the
hydraulic push rod in the released
position, the compensating port
in the master cylinder is open to
permit brake fluid to either
return from the brake system to
the fluid reservoir or enter the
brake system from the fluid
reservoir to compensate for ex-
pansion of/or loss of fluid from
the brake system.
b. Applied Position
As the brakes are applied by the
driver, the valve operating rod
and air valve plunger move to the
left in the power piston assembly
to compress the valve r e t u r n
spring and bring the floating con-
trol valve into contact with the
vacuum port seat in the valve
housing to "CLOSE" the vacuum
port. Any additional movement of
the valve operating rod in the
applied direction, moves the air
valve plunger away f r o m the
floating control valve to "OPEN"
the atmospheric port and admit
atmosphere through the air filter
and passages to the chamber at
the right of the power piston.
With the left side of the power
piston exposed to vacuum, and
atmospheric pressure on the right
side, a force is developed to move