Chevrolet Light Duty Truck 1973 Service Manual page 330

5 -6 4 BRAKES
As the power piston travels forward, the master cylinder
piston rod carries the master cylinder primary piston
further into the bore of the master cylinder. The force
on the master cylinder primary piston spring forces the
secondary piston to move forward. As the primary seal,
on both the master cylinder primary and the secondary
pistons pass the compensating ports in the bore,
hydraulic pressure will build up in the lines to the front
and rear brakes. As the pressure builds up on the end of
Fig. 8 9 -Typical Circuit - Applying
the master cylinder piston, the hydraulic reaction plate is
moved off its seat on the power piston and presses
against the reaction levers. The levers, in turn, swing
about their pivots and bear against the end of the air
valve operating rod assembly. In this manner approx­
imately 30% of the load on the hydraulic master cylinder
piston is transferred back through the reaction system to
the brake pedal. This gives the operator a feel, which is
proportional to the degree of brake application.
LIGHT DUTY TRUCK SERVICE MANUAL