Vacuum Brake Booster: Description and Operation

Brake Booster and Brake Cylinder, Assembly Overview

Note:
^ Brake master cylinder and brake booster can be replaced as individual components.
^ The Fig. shows the brake booster of the ABS/ESP Mk 20 system.
^ With the ABS/ESP Mk 60 system the components -Item 2, 14 and 15- as well as the magnetic coil for brake pressure, in brake booster N247 on the brake booster and the brake pressure release solenoid F84 are missing.





1 - Brake booster

^ Functional check

- With engine switched off, depress brake pedal firmly several times (to exhaust the vacuum in the unit).
- Now depress brake pedal with average foot pressure, hold and start engine. If the booster unit is working properly, the pedal will give slightly under foot (booster assistance becomes effective).

^ The vacuum required is taken from the intake manifold.

2 - Bracket

3 - Cap

^ With integrated float and brake fluid warning indicator

4 - Brake fluid reservoir

^ Moisten with brake fluid and press into sealing plugs in brake master cylinder
^ With warning contact for brake fluid level (phased introduction)
^ Remove brake fluid reservoir to replace warning contact

5 - Sealing plug

^ Moisten with brake fluid and press into brake master cylinder
^ Press brake fluid reservoir into sealing plugs.

6 - Retaining pin

^ For brake fluid reservoir

7 - Brake master cylinder

8 - Mounting plate

^ Different versions available

9 - Nut - 4 Nm

^ Self-locking
^ Replacing each time after removing

10 - Heat shield

11 - Brake Pressure Sensor 2 G214

12 - Brake Pressure Sensor 1 G201

13 - Seal

^ Always replace

14 - Electrical connector

15 - Electrical wire

16 - Vacuum line

^ With non-return valve

17 - Sealing plug

18 - Gasket

^ For brake booster

19 - Nut - 28 Nm

^ Self-locking
^ Replacing each time after removing

20 - Boot

^ If unusually loud suction noise is heard on braking, check proper positioning of inner boot.





- Checking inner boot of brake booster
- Remove storage compartment on drivers side.
- Check whether bellows have slipped out of annular groove as shown in this figure.
- Remove brake-pedal mounting bracket to insert boot.





Checking non-return valve

^ Non-return valve - A - should be open and allow passage of air in direction of - arrow - (blow into valve).
^ Valve should remain closed in opposite direction and not allow air to pass.

Note:
^ Pay attention to correct installation position.