Servicing the Bendix Hydrovac

Double Hydraulic Line System

Where a low initial pedal pressure is required, a Hydrovac unit (second series) which has two hydraulic lines connected to the vehicle cylinder, as
shown in figs. 37 and 38, can be used.

One line is connected between the master cylinder check valve outlet “A”, Fig. 34, and the slave cylinder port of the ilydrovac, Fig. 37. This line supplies fluid through the slave cylinder to the wheel cylinders.

Another line is connected between an outlet in the vehicle master cyl inder located behind its check valve, and a port in the control valve directly behind the valve piston. The sole purpose of this second line is to operate the control valve, and in doing so, the residual line pressure in the hy draulic brake system is eliminated from the control valve, thus requiring much less initial pedal pressure to operate the control valve, yet the original residual line pressure in the system through the other line is maintained.
In the released position, this second line is open to atmosphere through the master cylinder vent.

In the first hydraulic line referred to above, there is a ball check valve located in the passage between the slave cylinder piston and the control valve. The purpose of this check valve is to permit hydraulic fluid to pass pass from the control valve into the chamber behind the slave cylinder piston whenever the pressure within the former is greater than the pressure within the latter. If this “by-pass” were not provided, a vibration or pedal “chatter” might be set up under certain conditions. This is because within the time required for the control valve to move from applied to released position during operation of the Hydrovac unit the fluid might be forced out of the control valve and into the master cylinder through port “B”, fig. 34. If the “by-pass” were incorporated without the ball check valve, it would obviously be impossible to maintain residual line pressure in the brake system because this pressure would escape through the passage and be vented into the master cylinder through port “B”. The check valve is therefore incorporated to maintain residual line pressure within the brake system facilitating bleeding and filling, and insuring proper performance of the system.

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