Hydraulic Braking System

Hydraulic braking system

Brake fluid reservoir with level switch




1. Chambers
2. Plastic floats.
The brake fluid reservoir comprises two chambers (1), one on each brake circuit, which are filled with brake fluid. In the event of a rupture in one of the brake circuits, only some of the brake fluid in the system can escape. The remaining brake fluid acts on the operating brake circuit.
The reservoir contains a level function for detecting brake fluid loss in the brake system. The plastic float (2) floats on the brake fluid. This closes the connector on the switch when the brake fluid level drops below the MIN level on the reservoir. The stop (brake) warning lamp lights on the driver information module (DIM). The float and the switch are integrated with the container and cannot be replaced.

Master cylinder

Normal braking




The brake pedal is pressed and the piston nearest the servo cylinder moves to the left. Pressure builds up in the primary circuit.
This pressure also moves the second piston to the left and pressure builds up in the secondary circuit.

Leakage in the primary circuit




The brake pedal is pressed and the piston nearest the servo cylinder is moved, but does not build up pressure due to damage in the primary circuit. The piston moves until it comes into contact with the piston in the secondary circuit. Only then can pressure build up in the secondary circuit. The brake pedal stroke is longer than during normal braking.

Leakage in the secondary circuit




The brake pedal is pressed and fluid in the primary circuit is pressed against the secondary circuit piston. The piston cannot build up pressure due to damage in the secondary circuit. It therefore moves to the left until it reaches the limit position in the master cylinder. Only then can pressure build up in the primary circuit.
The brake pedal stroke is longer than during normal braking.

Servo cylinder

Resting position




1. Max. negative pressure
2. Atmospheric pressure.
In the resting position, the servo cylinder components are in the position 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




1. Max. negative pressure
2. Atmospheric pressure.
3. 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 illustrated. 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.

Mechanical brake assistance (EBA)




1. Snap-ring
2. Rubber disc
3. Control housing
4. Ratio disc
5. Spring, Ball socket
6. Valve piston
7. Connecting device.
8. Bearing race
9. Guide sleeve
10. Seal
11. Ball socket
12. Ball bearing
13. Locking sleeve
14. Spring, Locking sleeve
15. Clock valve
16. Mounting bracket
17. Retainer
18. Spring, Clock valve
19. Gasket
20. Push rod spring
21. Spring
Mechanical brake assistance (EBA) is integrated with the servo cylinder. The regular function of the servo cylinder is not affected, although the design in the guide housing is affected slightly.

Partial braking position without mechanical brake assistance (EBA)




If the movement from the speed and force of the brake pedal does not exceed the designed locking movement, the locking sleeve (13) cannot go to its final position. The vehicle brakes without brake assistance.
The ball socket (11) can move axially throughout the entire movement of the valve piston (6) due to the ball bearings (12), which are free to move radially over the ramp on the ball socket.

Activation of the EBA function




When the switching movement is exceeded (sufficiently high speed and force from the brake pedal are required), the movement of the valve piston (6) moves the ball bearings (12) radially inwards over the ball socket (11) ramp. The locking sleeve (13), which is pressed by the spring (14), can therefore move to the right (in the illustration). The ball bearings are now locked and the EBA function is activated.

Full braking position without physical effect (EBA function activated)





When the EBA function is activated (the ball bearings locked and the connecting device in activated position), the reaction disc (2) force is transferred to the bearing race (8) and the guide housing (3) via the ball socket (11) and the ball bearings (12). The driver can now increase the brake pressure to fully depressed brake pedal (full pedal deflection) with no physical effort.

Release position mechanical brake assistance (EBA)




To reduce the brake pressure, the connecting device remains in activated position. The driver controls the reduction of the pressure via the pedal movement. In the same way as with a servo cylinder without brake assistance, this is achieved by closing the valve for atmospheric pressure and simultaneously opening the vacuum valve. Immediately before the resting position is achieved, the locking sleeve (13) comes into contact with the mounting bracket (16). The locking sleeve is then pressed back (to the left in the illustration) to its original position. The ball bearings are released and the connecting device is in passive mode once more. The EBA function is no longer activated. The locking sleeve (13) returns to its original position and the brake assistance function is terminated.