Hydraulic Brake Booster: Description and Operation

PURPOSE
The hydraulic brake booster provides support to the force of the pedal when the brake is actuated and thus reduces the amount of force which must be applied by the driver.





OPERATION
The pressure controlled flow regulator (power flow regulator) directs the fluid for the power booster (approximately 0.7 liters per minute) from delivery flow to the steering box and charges a pressure storage reservoir with a pressure of 36-57 bar. This does not reduce the effectiveness of the power assisted steering.

The hydraulic fluid, under pressure in the pressure storage reservoir (hydraulic reservoir), is available for booster operation whenever the brakes are applied.

If the power steering pump falls, sufficient reserve pressure will be stored In the hydraulic reservoir to be able to execute several braking actions with full brake boosting support. After consumption of this reserve pressure, the brakes can still be applied, but without boost support, and therefore will require considerably more force on the brake pedal.





Hydraulic Reservoir Charging Position
The upper chamber of the pressure reservoir is filled with nitrogen at the pressure of 22-24 bar. The lower chamber is charged with the fluid from the power assisted steering circuit.

The flow of the fluid from the power assisted steering circuit enters into the DS regulator (power flow regulator) through the connection "P".

The flow control piston (2) divides the flow of the fluid.

One-tenth of the flow is fed over to the switching valve (3) and the check valve (4) which allows the pressure reservoir to be charged up. The outlet from the pressure reservoir to the brake booster is via connection 'A".

Nine-tenths of the flow is directed to the steering box via the flow control piston (2) and connection "B".





Hydraulic Reservoir Fully Charged Position
The pressure reservoir (1) is fully charged when a pressure point of 52-57 bar is reached.
When the cutoff pressure is reached in the hydraulic reservoir the switching valve (3) closes the line going to the check valve and at the same time opens the return line to the fluid reservoir and relieves the pressure on the back of the flow control piston (2) The full pressure of the power steering pump presses the piston back and all the flow is now available for the power steering.





Brake Booster Relaxed Position
The servo piston is held in the relaxed position by the return spring (4). Pressure can only reach the closed regulating slit (1). The inlet port of the control piston (2) is also closed and both sides of the servo piston are without pressure, linked via outlet (3) through the return line to the reservoir.





Brake Booster Partial Braking
By operating the brake pedal, a force is directed via the pushrod (5) to the control piston (6). The control piston moves against the spring pressure (9). The outlet port (3) to the return line is closed. The inlet slit (1/2) is opened. Pressure from the pressure storage reservoir can now enter and act on the rear of the servo piston (7) and on the face of the pushrod (5) until a balance between the pedal and servo force and the return pressure from the master cylinder is achieved.

Any slight change of the pedal pressure will cause an immediate reaction to restore the balance.

The pressure on the pushrod piston (5) opposes pedal pressure to provide a "feel" in the peda1 proportional with the pressure applied to the brakes.





Brake Booster Full Braking
In this position the transfer bores (1-2) are fully open, the control piston (6) is on its stop (11).

The full system pressure is applied behind the servo piston (7). Therefore the maximum possible assistance is obtained. This means that the booster is fully engaged. Further increase in brake pressure is only possible by increased "pedal pressure.