Buick LeSabre 1965 Service Manual page 550

9 - 2 8 POWER BRAKES
BRAKES
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/ CONTROL VALVE
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HERE
its snap ring rests against the
power piston. As it returns, the
air valve pushes the floating con-
trol valve off its seat on the
power piston.
The air valve seating on the float-
ing control valve has shut off the
outside air source. When it lifts
the floating control valve from its
seat on the power piston, it opens
the space to the right of the power
piston to the vacuum source.
Since both sides of the power pis-
ton are now under vacuum, the
power piston return spring will
return the piston to its released
position against the rear housing.
As the power piston is returned,
Figure 9-18—Power Brake Unit - Holding
the hydraulic master cylinder
piston moves back, and the fluid
from the wheel cylinders flows
back into the master cylinder
through the check valve.
If the brake pedal is released
quickly, the master cylinder pis-
ton immediately returns to the
released position. If the fluid in
the lines cannot return as quickly
as the piston, compensation is
provided for by the flow of fluid
from the space between the pri-
mary cup and the secondary seal
through the holes in the piston.
The excess fluid in the system
can flow back to the fluid reser-
voir through the small by-pass
hole in the master cylinder bore
after the brake is released.
9-15 POWER BRAKE
UNIT TROUBLE
DIAGNOSIS
The same types of brake trouble
are e n c o u n t e r e d with power
brakes as with standard brakes.
Before checking the power brake
system for the source of trouble,
refer to the trouble diagnosis of
standard brakes in Section 9-B.
After these possible causes have
been eliminated, check for the
cause as outlined below: