The hydraulic booster applies force to the master cylinder when the brake pedal is applied. Brake pedal movement controls the fluid flowing through the booster head.

The fluid from the hydraulic brake booster pump enters the booster at the inlet port. The fluid flows through the control valve and the seat and exits through the outlet port and the flow switch before returning to the hydraulic brake booster pump.

The hydraulic booster cylinder rod attaches to a piston that contains the control valve seat. The assembly moves when pressure is applied to the brake pedal. As the control valve moves closer to the control valve seat in the piston, the fluid flow is restricted and the pressure builds on one side of the piston. The pressure overcomes the return spring and moves the piston to a balanced position. The piston is balanced when the pressure on each side of the return spring equals the pressure of the hydraulic fluid flowing through the control valve and the seat. The piston pushes the cylinder rod and applies pressure to the master cylinder as the piston moves.

A relief valve inside the booster head limits the pressure to 4826 kPa (700 psi). The limiting of the pressure in the booster head limits the brake line pressure at the master cylinder to 13455 kPa (1950 psi). The pressure level provides good braking without damaging the brake pipes or the hoses. The relief valve allows fluid to bypass the piston when actuated. Fluid constantly flows through the booster head when the engine is running.

The Electrohydraulic (EH) pump is used as a backup to the belt driven power steering pump. The EH pump turns on when the fluid stops flowing through the booster head. The EH pump is a small hydraulic pump that mounts to the bottom of the booster head.

The EH pump draws fluid from the low pressure side of the booster head piston. The EH pump delivers fluid to the high pressure side. The normal output of the EH pump is 2070-2415 kPa (300-500 psi) which is about half that of the power steering pump.

A flow switch in the booster head outlet port senses the fluid flow. The flow switch controls a relay for the EH pump. A pedal switch also activates the EH pump when the brake pedal is applied and the engine is not running.

Stationary pipes and flexible hoses carry fluid throughout the hydraulic brake booster system. The stationary pipes and the flexible hoses connect the booster head to the hydraulic brake booster pump and the fluid reservoir.

Refer to the instructions enclosed with the service repair kit for overhaul information on the Bendix® brake booster. Contact the Bendix® Helpline at 1-800-AIR-BRAK (1-800-247-2725) for additional overhaul information.

Notice: Hydraulic brake systems use two distinct and incompatible fluids. Power steering fluid is used in the hydraulic brake booster system. Brake fluid is used in the master cylinder and brake pipes. Use extreme care when selecting brake system fluids, or seal damage can result. Refer to General Information to select the correct fluid.

The hydraulic booster system uses no special fluids. However, ensure the use of the correct fluids. The master cylinder and the brake system use brake fluid. The hydraulic brake booster pump uses power steering fluid.

Contaminated fluid causes swelling and deterioration of the rubber parts. The swelling and deterioration leads to reduced brake performance and the loss of braking capability. Inspect the condition of the fluid at regular intervals and note any unusual consistency, color and signs of contaminants in the fluid.

Notice: Power steering fluid and brake fluid cannot be mixed. If brake seals contact power steering fluid or steering seals contact brake fluid, seal damage will result.

Do not reuse the brake system fluids Always discard the used fluid. Do not mix the power steering fluid with the hydraulic brake fluid. Flush the hydraulic booster system with clean power steering fluid if contamination occurs.

Do booster component bench servicing in a clean work area separate from the brake servicing area. Wash your hands before changing between the brake or the booster work area. Do not use the same containers for the fluids.