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 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 power steering 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 power steering pump is a belt or a gear driven vane-type pump that sends fluid to the booster head and the power steering system. A fluid reservoir mounts to the rear cab support.

The hydromax system is an auxilliary brake assist system that ensures brake assist is available if the engine stalls or if there is a malfunction in the power steering/brake assist system. The hydromax system consists of the following components.

The electronic brake control module (EBCM) monitors the fluid flow switch and energizes the hydromax motor control relay when hydromax assist is required. When the fluid flow switch is closed, the hydromax motor turns ON whenever the EBCM detects that the brake is applied. The EBCM periodically perfoms electrical diagnostic tests on the fluid flow switch, hydromax motor, hydromax motor control relay and the stop lamp switch. If a hydromax failure is detected, the EBCM sets a DTC and may turn on a warning indicator and a fast rate audible chime.

A flow switch in the booster head outlet port senses the power steering 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 power steering pump and the power steering gear.