When the driver moves the brake pedal pushrod, it transmits movement through the power unit to the master cylinder piston, to apply the brakes.
It also operates a control valve that admits air, at atmospheric pressure, to the rear of the unit. How it works depends on the position of the pushrod.
A hose connects the intake manifold to a vacuum check valve on the power unit. With the engine running, the vacuum in the intake manifold is used to evacuate the power unit. This valve is held off its seat and a vacuum is produced in both chambers of the unit.
The chambers are separated by a flexible rubber diaphragm attached to the diaphragm plate. It is held in the off position by a diaphragm return spring. The master cylinder pushrod and the control valve assembly are centrally located on each side of the plate.
As the brakes are applied, the pedal pushrod and plunger move forward in the diaphragm plate. This brings the control valve into contact with the vacuum port seat. It closes the vacuum port, sealing off the passage connecting the chambers. Further movement of the pushrod and plunger moves the air valve away from the control valve to open the atmospheric port. Air at atmospheric pressure comes into the air filter and passages, and enters the chamber at the rear of the diaphragm. The difference in pressure now on both sides of the diaphragm moves the diaphragm plate forward, and it takes the master cylinder pushrod with it.
Hydraulic pressure builds up in the brake system to operate the brakes. As pressure rises, a counter-force acts through the master cylinder pushrod and the reaction disc. This counter force acts against the plunger and pedal pushrod. It tends to move the plunger slightly to the rear, and it closes off the atmospheric port.
If the vacuum source is interrupted then, as the pedal is pushed down, the pedal pushrod and plunger assembly come in contact with the reaction disc. This forces the master cylinder pushrod forward, to operate the brakes. The pedal force needed then is much greater than with vacuum assistance.
