Bendix Hydro-Boost brake unit cutaway diagram showing valves, pi…

Hydro-Boost Generating Pressure
During normal power operation, the brake pedal is applied as shown by input rod travel A. The movement of the ratio lever operates the spool valve creating hydraulic pressure due to the restriction at land 3. Fluid traverses land 2 and flows through the center of the spool into the boost cavity. This pressure acts against the power piston to give the power assist and against the reaction rod to provide the effort level of the brake pedal.

If, while braking, fluid pressure is required for steering, pump pressure will rise and the spool valve will shift in an open direction allowing more fluid to flow to the steering gear. Even under extremes of braking, the fluid flow to the steering gear is fully sufficient for steering the vehicle.

The enlarged view of the spool valve shows the relationship of the valve lands during normal power operation. Land 1 is closed blocking the oil flow to the reservoir. Land 3 meters the flow of oil from the supply pump to the steering gear and generates the hydraulic pressure that is metered across land 2 to provide assist to the brakes.

The enlarged view of the valve shows the same operation except pressure is generated by the steering gear and is also being used for braking. During this mode of operation, the spool travel is slightly less than the first case.

For disassembled photos, see this post.

Owner caption January 2006 · Photo 1155 of 5000