In the applied position hydraulic pressure from master cylinder is applied to the control valve piston “G” moving vacuum valve “A” to the closed position and opening atmos pheric valve “B”. This admits atmosphere to cylinder chambers “C’’ and ‘‘E” while vacuum is maintained in chambers “D” and ‘‘F’.

Should the vacuum system become inoperative due to damaged lines or other accidental cause, the power cylinder pistons return to released position and the vehicle hydraulic system automatically returns to physical operation. Under this condition the power cylinder pistons remain at rest; the hydraulic fluid flows through the hydraulic piston, through the slave cylinder and to the wheel cylinders. Therefore, there is no drag on the physical braking system.

The Hydrovac has been called a “one - unit” vacuum power brake system because it combines in one unit the familiar elements of control valve and power cylinder with necessary connections. The control valve is actuated by hy draulic pressure from the vehicle master cylinder; the power cylinder of the Hydrovac actuates a hydraulic cylinder which

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