Chevrolet Light Duty Truck 1973 Service Manual page 329

APPLYING (Figs. 88 and 89)
As the pedal is depressed, the valve operating rod (push
rod) carries the air valve away from the floating control
valve. The floating control valve will follow until it is in
contact with the raised seat in the power piston insert.
When this occurs, vacuum is shut off to the rear of the
Fig. 88-Typical Power Unit - Applying
power piston, and air under atmospheric pressure enters
through the air filter and travels past the seat of the air
valve and through a passage into the housing at the rear
of the power piston. Since there is still vacuum on the
front side of the power piston, the atmospheric air
pressure at the rear of the piston will force the power
piston to travel forward.
BRAKES
VACUUM
REACTION
PLATE
AIR VALVE
UNSEATED
AIR VALVE
REACTION
LEVERS
ATMOSPHERIC
PRESSURE
LIGHT DUTY TRUCK SERVICE MANUAL
5 -6 3
FLOATING
CONTROL
VALVE