Operation

Upon application of the brakes, the valve rod and plunger move to the left in the power diaphragm to close the vacuum port and open the atmospheric port to admit air through the air cleaner and valve at the rear diaphragm chamber. With vacuum present in the rear chamber, a force is developed to move the power diaphragm, hydraulic pushrod and hydraulic piston or pistons to close the compensating port or ports and force fluid under pressure through the residual check valve or valves and lines into the front and rear wheel cylinders to actuate the brakes.
As pressure is developed within the master cylinder a counterforce acting through the hydraulic pushrod and reaction disc against the vacuum power diaphragm and valve plunger sets up a reaction force opposing the force applied to the valve rod and plunger. This reaction force tends to close the atmospheric port and reopen the vacuum port. Since this force is in opposition to the force applied to the brake pedal by the driver it gives the driver a ``feel'' of the amount of brake applied. The proportion of reactive force applied to the valve plunger through the reaction disc is designed into the Master-Vac to assure maximum power consistent with maintaining pedal feel. The reaction force is in direct proportion to the hydraulic pressure developed within the brake system.