Buick LeSabre 1965 Service Manual page 562

9 - 4 0 POWER BRAKES
BRAKES
POWER PISTON
POWER PISTON
RETURN SPRING
ATMOSPHERIC
PRESSURE
FLOATING
CONTROL VALVE
OPERATING ROD
AIR VALVE
RETURN SPRING
Figure 9-36—Power Brake - Released Position
the power piston assembly, hy-
draulic push rod and hydraulic
piston to the left to close the
compensating port and force hy-
draulic f l u i d under p r e s s u r e
through the residual check valve
and brake tubes into the brake
wheel cylinders.
As hydraulic pressure is de-
veloped within the master cylin-
der, a counter force (to the right),
acting through the hydraulic push
rod and rubber reaction disc, sets
up a reaction force against the
power p i s t o n and air v a l v e
plunger. The rubber reaction disc
distributes the pressure between
the power piston assembly and
the valve plunger in proportion to
their respective contact areas.
The pressures acting against the
air valve plunger and valve oper-
ating rod tend to move the air
valve plunger slightly to the right
in relation to the power piston
and valve housing assembly to
close off the atmospheric port.
Since this counter force or reac-
tion force is in direct proportion
to the hydraulic pressure de-
veloped within the brake system,
the driver is able to maintain a
"feel" of the degree of brake
application attained.
c. Holding Position
During application of the brakes,
the "reaction" against the air
valve plunger is working against
the driver to close the atmos-
pheric port. With both atmos-
pheric and vacuum ports closed,
the power brake is said to be in
the "lap" or holding position.
When both valves are closed or
"lapped off", any degree of brak-
ing application attained will be
held until either the atmospheric
port is reopened by an increase
in pedal pressure to further in-
crease the brake application or
by a decrease in pedal pressure