Servicing the Bendix Hydrovac

The. operation of the Hydrovac (second series), with the exception of Double Hydraulic Line System, is identical with the operation of the Hydrovac (first series) as described on pages 3, 5, 6 and 7.
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In the released position, the vacuum valve “A”, Fig. 35, is open, atmospheric valve “B”, Fig. 35, is in closed position and vacuum is present on both sides of power cylinder pistons, chambers C. D. F, and F, Fig. 35
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In the applied position, hydraulic pressure from the master cylinder is applied to the control valve piston “G” moving the valve diaphragm and seat up against the vacuum valve “A”, closing this valve and opening atmospheric valve “B”. This admits atmosphere to the Hlydrovac chambers 'C' and 'E' while vacuum is maintained in chambers “D” and “F”, Fig. 36.

In the holding position, when the brake pedal is held partially applied and physical pressure is constant, the master cylinder line pressure at point 'G' remains constant as both valves are closed. If pressure on the pedal is slightly reduced, then vacuum valve “A” opens reducing atmospheric pressure in sections “C” and “E”.

Thus, the force of the vacuum pistons against slave cylinder piston is reduced. The brakes therefore relaese to any desired degree, depending on amount by which the pressure on the foot pedal has been reduced, - and complete brake release is obtained when pressure has been entirely removed from the brake pedal.

As is the case with the Hydrovac (first series), 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 slave cylinder hydraulic piston, through the slave cylinder to the wheel cylinders. Therefore, there is no drag on the physical braking system.

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