Servo Cylinder
Resting position
- Max. negative pressure
- Atmospheric pressure.
In the resting position, the servo cylinder components are in the position shown in the illustration. 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 in the illustration. 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 in the illustration.
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 brakes.
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 partial braking position and the valve seat on the moved guide housing can now close the connection between the rear side of the diaphragm and atmospheric pressure. No further pressure is built up on the diaphragm. The force does not increase but is now as large as the hydraulic back pressure in the master cylinder. A certain extension of the piston will then occur. The piston seat leaves the valve. More air can flow in and greater brake application can be obtained 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 at it will go and the servo cylinder reinforces the braking force applied by the driver to the maximum.
If the force on the pedal is increased the force of the valve piston on the center of the reaction disc increases.
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 shown in the illustration. The new balance position has been achieved. If the pedal is completely released, all the servo cylinder 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 servo cylinder operating as an extended push rod. As no servo effect is obtained in this case, greater pedal force is required.