Brake Master Cylinder: Description and Operation

Brake Booster/Master Cylinder Assembly Overview

• Use only new brake fluid. Observe information on brake fluid reservoir!





1 Brake pedal assembly

2 Self locking hex nut, 25 Nm

• Always replace after removal.

3 Gasket

• For brake booster.

4 Brake booster

• Vacuum is supplied from the intake manifold.

• Some vehicles with automatic transmissions are equipped with a brake system vacuum pump. Refer to => [ Brake System Vacuum Pump ] Brake System Vacuum Pump.

• On vehicles with a hydraulic brake booster, a vacuum sensor is installed. Refer to => [ Brake Booster Vacuum Sensor ] Brake Booster Vacuum Sensor.

• Function test:

- With engine switched off, press the brake pedal firmly several times (to deplete vacuum).

- Press and hold the brake pedal with average foot pressure and start the engine. If the brake booster is working properly, the pedal will be felt to give noticeably under foot (booster assistance becomes effective).

• Replace as a unit in the event of malfunction (previously, check all vacuum lines).

• Removing and installing, refer to => [ Brake Booster ] Brake Booster.

5 Seal

6 Brake booster vacuum sensor (G483)

• Only on vehicles with the hydraulic brake booster.

• Removing and installing, refer to => [ Brake Booster Vacuum Sensor ] Brake Booster Vacuum Sensor.

7 Brake master cylinder without brake light switch

• Cannot be serviced. Replace as complete unit if malfunctioning.

• Removing and installing, refer to => [ Master Cylinder ] Service and Repair.

8 Brake master cylinder with brake light switch

• Cannot be serviced. Replace as complete unit if malfunctioning.

• Removing and installing, refer to => [ Master Cylinder ] Service and Repair.

9 Brake light switch (F)

• Includes brake pedal switch.

• On vehicles before 11.06.05, the brake light switch is located on the brake pedal. Refer to => [ Brake Pedal Assembly Overview ] Brake Pedal Assembly Overview.

• Removing and installing, refer to => [ Brake Light Switch, from 11.06.05 ] Service and Repair.

10 Inner Torx bolt, 5 Nm

11 Heat shield

12 Self locking hex nut, 25 Nm

• Always replace after removal

13 Brake line, 14 Nm

• Master cylinder/secondary piston circuit to hydraulic unit.

14 Brake line, 14 Nm

• Master cylinder/primary piston circuit to hydraulic control unit.

15 Sealing plugs

• Lubricate with brake fluid and press into brake fluid reservoir.

16 Brake fluid reservoir

17 Cap

18 Sealing plugs

• Connection for vacuum hose.

19 Vacuum hose

• Insert into brake booster.

20 Brake booster pressure sensor (G294)

• on vehicles with Fuel Straight Injection (FSI) engine without hydraulic brake vacuum booster.

• Removing and installing, refer to => [ Brake Booster Pressure Sensor ] Service and Repair.