Chevrolet Light Duty Truck 1973 Service Manual page 331

HOLDING (Figs. 90 and 91)
When the desired pedal pressure is reached, the power
piston moves forward until the floating control valve,
which is still seated on the power piston, again seats on
the air valve. The power piston will now remain
stationary, until either pressure is applied or released at
the brake pedal. As the pressure at the pedal is released,
the air valve spring forces the air valve back to its stop
on the power piston. As it returns, the air valve pushes
the floating control valve off its seat on the power piston
insert. The air valve seating on the floating control valve
has shut off the outside air source. When it lifts the
floating control valve from its seat on the power piston
insert, it opens the space at the rear of the power piston
to the vacuum source.
Fig. 90--Typical Power Unit - Holding
Since both sides of the power piston 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, the master cylinder
primary and secondary pistons move back, and the fluid
from the wheel cylinders flows back into the master
cylinder. If the brake pedal is released quickly, the
master cylinder primary and secondary pistons immedi­
ately return to the released position. If the fluid in the
lines cannot return as quickly as the pistons, this is
compensated for by the flow of fluid from the space
between the primary and secondary seals through the
compensating holes in the pistons. The excess fluid in the
system can flow back to the fluid reservoirs through the
small compensating ports in the master cylinder bore.
VACUUM
REACTION
PLATE
FLOATING
CONTROL
AIR VALVE
REACTION
LEVERS
ATMOSPHERIC
PRESSURE
LIGHT DUTY TRUCK SERVICE MANUAL
BRAKES 5 -6 5
VALVE
AIR VALVE
SEATED