K 2500 Truck 4WD V8-379 6.2L DSL VIN C FI (1989)
The hydraulic pushrod operates against the master cylinder piston. A split system (tandem piston) type master cylinder is incorporated into some units.
The front half of the master cylinder in the split system operates the rear brakes while the rear half of the master cylinder operates the front brakes.
A vacuum check valve, attached to the front vacuum chamber and connected to the intake manifold, traps vacuum in the power unit at the highest level
of vacuum.
OPERATION
As the brakes are applied by the driver, the valve operating rod and control piston move forward in the power piston assembly to compress the valve
return spring and bring the poppet valve into contact with the vacuum valve seat in the valve housing to close the vacuum post. Any additional movement
of the valve operating rod in the applied direction moves the control valve away from the poppet valve to open the atmospheric port and admit air
through the air filter and passages to the chamber at the right of the vacuum power piston assembly. With vacuum on the left side of the diaphragm and
atmospheric pressure on the right side of the diaphragm, a force is developed to move the vacuum power piston assembly, hydraulic pushrod, and
hydraulic piston to the left to close the compensating port and force hydraulic fluid under pressure through the residual check valve and brake tubes into
the brake wheel cylinders.
