Buick LeSabre 1965 Service Manual page 549

BRAKES
POWER BRAKES 9 - 2 7
VACUUM
REACTION
PLATE
REACTION
LEVERS
FLOATING CONTROL
VALVE
AIR FILTERS
AIR VALVE
ATMOSPHERIC
PRESSURE HERE
As the power piston travels to the
left, the piston rod carries the
master cylinder piston into the
bore of t h e master cylinder.
After the master cylinder piston
cup passes the by-pass hole, hy-
draulic pressure starts to build
up in the hydraulic system. As
the pressure builds up on the end
of the master cylinder piston, the
hydraulic reaction plate is moved
off its seat on the reaction re-
tainer and presses against the
reaction levers. The levers, in
turn, swing about their pivots and
bear against the end of the air
valve-push rod assembly.
In this manner, approximately
30% of the load on the piston is
Figure 9-17—Power Brake Unit - Applying
transferred back through the re-
action system to the brake pedal.
This gives the operator a feel,
which is proportional to the de-
gree of brake application.
In case of vacuum source inter-
ruption, as the pedal is pushed
down, the end of the air valve
contacts the reaction levers and
forces them, in turn, against the
hydraulic reaction plate. Since
the hydraulic reaction plate is
fastened to the piston rod, it
f o r c e s the piston rod against
the master c y l i n d e r piston,
which builds up the hydraulic line
pressure.
The pedal pressure required for
a manual application, such as de-
scribed, is considerably greater
than with vacuum assist.
(3) Holding Position
When the desired pedal pressure
is reached, the power p i s t o n
moves to the left until the floating
control valve, which is still seated
on the power piston, again seats
on the air valve. The power brake
will now remain stationary, until
either pressure is applied or re-
leased at the brake pedal. See
Figure 9-18.
(4) Releasing Position
As the pressure at the pedal is
released, the air valve spring
forces the air valve back until