The end of the piston push rod fits into a tapered valve which seats against the hydraulic piston. A passage through the hydraulic piston allows fluid to pass through when the valve is off the seat, that is, when the power cylinder pistons are in released position.

When the driver first steps upon the brake pedal, the passage through the hydraulic piston is open because the power cylinder pistons are in released position. As hydraulic pressure builds up, the control valves move into applied position, causing the power cylinder pistons to act. The first movement of the piston rod closes the passage through the hydraulic piston; further movement in the applied direction forces the hydraulic piston further into the cylinder and thus applies the brakes. Note that during brake application the hydraulic force from the vehicle master cylinder, necessary to operate the control valve, is automatically added to the force of brake application, since it also acts upon the Hydrovac hydraulic piston.

In the released position the vacuum valve “A” is open, atmospheric valve “B” is in the closed position. Vacuum is present on both sides of power cylinder pistons (chambers “C”, “D”, ‘‘E and ‘F’’.)

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