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Servicing the Bendix Hydrovac

(First Series)

The Power Cylinder
The power cylinder of the Hydrovac is divided into two equal compartments, * each of which is provided with a piston. The two pistons work on one piston rod so that the effect is that of two cylinders placed end-to-end with pistons connected in tandem.
This arrangement gives double the effective piston area and consequently twice the power of a single cylinder of the same diameter. Each cylinder compartment is provided with the necessary seals and passages to operate on the vacuum suspended principle.
A helical spring serves to return the pistons to released posi tion when the air pressures on both sides of each piston are balanced.

Control Valve.
Hydraulic pressure from the master cylinder of the vehicle acts upon the hydraulic plunger in the control valve and forces the diaphragm toward the valves. This movement of the diaphragm moves the valves into the applied position, thereby allowing the hydrovac cylinder pistons to move.
As air is admitted into the valve compartment below the diaphragm, the diaphragm is forced against the plunger until the valves are brought into the holding position. The diaghram, therefore, exerts a reactionary force against the plunger which is exactly proportional to the degree of brake application. This reactionary force not only brings the valves into the holding position but it is hydraulically transmitted to the foot pedal to give pedal feel or reactionary control.

Hydraulic Slave Cylinder
The hydraulic or “slave” cylinder of the Hydrovac unit is provided with a rubber cup and piston similar to the vehicle master cylinder. The piston rod from the power cylinder forces this piston and cup further into the hydraulic slave cylinder. In that way, fluid is forced to the vehicle wheel cylinders to apply the brakes.

*Certain Hydrovac installations which require less power, use a design with one cylinder and piston; otherwise the units work a described.

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