Power Brake Booster
Overview of the passive power brake booster
| Number | Designation | Number | Designation |
| 1 | Vacuum hose connection | 7 | Valve piston |
| 2 | Front push rod | 8 | Air cleaner (ACL) |
| 3 | Membrane spring | 9 | Rear push rod |
| 4 | Rubber membrane | 10 | Push rod spring |
| 5 | Membrane support | 11 | Guide housing |
| 6 | Reaction disc |
The vehicle is equipped with a double power brake booster. The power brake booster is located between the brake pedal and the master cylinder and is actuated directly by the brake pedal. It ensures that less pedal force is required when braking with the aid of vacuum from the vacuum pump and via the intake manifold for the engine. The check valve located by the vacuum pump prevents air from flowing back to the power brake booster.
Resting position
- Max. negative pressure
- Atmospheric pressure
In the resting position, the power brake booster components are in the positions illustrated. The push rod spring keeps the push rod and the jointed valve piston mounted on this pressed to the right. The movement is restricted by the stop washer. The valve piston holds the valve lifted out of the seat in the guide housing. The air duct is closed and the vacuum duct exposed. The same negative pressure is therefore present on both sides of the membrane. The membrane and the guide housing are pressed to the right-hand limit position by the membrane spring.
When the brake pedal is pressed down, the rear push rod and the valve piston are moved to the left (forwards). The valve spring causes the valve plate to follow until it reaches the seat in the guide housing. The connection between the front and reverse of the membrane is closed. As the piston continues to move forward, its movement is transferred via the reaction disc and the front push rod to the brake fluid in the master cylinder.
Partial and full braking
- Max. negative pressure
- Atmospheric pressure
- Negative pressure
When the driver presses the brake pedal, the valve piston moves to the left as illustrated. The seat for the valve piston leaves the valve plate and the connection opens between the reverse of the membrane and the center of the valve assembly (which is at atmospheric pressure). Air at atmospheric pressure flows in behind the membrane. There is negative pressure on the front of the membrane. A difference in pressure is built up. The difference in pressure generates a force on the membrane, causing the membrane to move to the left as illustrated.
The membrane is installed on the guide housing, which therefore follows the movement of the valve piston. The guide housing force is transferred to the front push rod through the outer part of the reaction disc. Along with the force from the brake pedal, which presses on the inner part of the reaction disc, the front push rod presses the brake fluid out of the master cylinder to the wheel brakes. The vehicle is braked.
During normal braking, when the driver presses the brake pedal to a certain level and holds the pedal in this position, the following occurs:
The valve piston stops in the partial braking position and the valve seat on the moved guide housing can now close the connection between the reverse of the membrane and atmospheric pressure. No additional pressure is built up on the membrane. The force does not increase, but is now equal to the hydraulic counter pressure in the master cylinder.
If the force on the pedal is increased, the valve piston force on the center of the reaction disc increases. Some displacement of the piston will take place. The piston seat leaves the valve. More air can flow in and increased braking is achieved until the new balance position is achieved.
During full braking, when the driver presses the brake pedal all the way down, the following occurs: Air from atmospheric pressure flows behind the membrane until the maximum difference in pressure has built up. The membrane moves as far as it will go and the power brake booster reinforces the braking force applied by the driver to the maximum.
If the pedal force is reduced, the center of the reaction disc is pressed from the seat in the guide housing. The spaces on either side of the membrane are then linked to each other. The pressure is equalized, the guide housing is pushed back by the spring force and the counter force on the front push rod. Braking reduces. The valve piston can revert to the position illustrated. The new balance position has been achieved. If the pedal is completely released, all the power brake booster components return to the resting position and the brakes are released.
In the event of a fault in the vacuum supply, braking can still take place by means of the power brake booster operating as an extended push rod. As no servo assistance is obtained in this case, greater pedal force is required.