Replacing brake fluid and bleeding the brake system (Audi A4 B7)

General information

Warning: Contact with brake fluid is hazardous to health. Do not suck it out with your mouth through a hose. Store brake fluid only in a container that is out of the reach of children.

1. Brake fluid has the property of absorbing moisture, as a result of which its boiling point decreases over time, which, under significant load on the brakes, can lead to the formation of steam bubbles in the brake lines and reduce the effectiveness of the brakes.

2. To replace the brake fluid, you will need the help of an assistant, but this procedure can be carried out without outside help using a special device - see the instructions supplied with such a device (the pressure created by the device must not exceed 2 bar). When bleeding the brake system, the vehicle must be in a horizontal position to properly monitor the brake fluid level in the reservoir.

Replacing brake fluid and bleeding the brake system and clutch

3. Turn off the ignition.

4. Mark the current brake fluid level on the tank wall with a felt-tip pen to prevent it from being exceeded when replacing the brake pads. Unscrew the plug (arrow on the illustration) brake fluid reservoir and pump out approximately 100 ml of brake fluid from the reservoir, leaving approximately 1 cm of liquid.

Note: Do not remove all the fluid from the reservoir, otherwise air will enter the hydraulic drive of the brake system.

5. Remove the brake fluid reservoir cap. If a bleeding device is used, connect it to the reservoir instead of the cap. Otherwise, fill the reservoir with fresh brake fluid to the "MAX" mark and maintain this level while performing the following procedures.

6. On models with manual transmission, remove the cap from the clutch hydraulic bleed nipple on the clutch slave cylinder (see illustration), connect a hose to the nipple and drain approximately 100 ml of brake fluid from the clutch hydraulic system. Tighten the nipple, close it with a cap and press the clutch pedal several times.



7. Thoroughly clean and remove the protective caps of the bleed nipples on all brake mechanisms (see Chapter 9). Attach one end of a clean, transparent hose to the brake bleeder nipple on the front left wheel (the nipple is located on the brake wheel cylinder), and insert the other end into a container for collecting brake fluid. Pour some brake fluid into the container and constantly ensure that the end of the hose is immersed in it. Also, the container should be raised at least 30 cm above the bleed nipple to ensure pressure and prevent air from entering it.



8. When using a bleeding device, when you unscrew the bleeding nipple, the brake fluid comes out by itself. Bleed approximately 200 ml of brake fluid, tighten the nipple and go to paragraph 11

9. Ask an assistant to press the brake pedal sharply 3-5 times at intervals of 2:3 sec and release the nipple half a turn with the pedal depressed. After the brake pedal hits the floor, tighten the nipple. Remove your foot from the pedal. Repeat this process to pump out the old brake fluid (until a new one appears, which can be recognized by its light color).

Note: If the liquid layer at the bottom of the tank is less than 10 mm, the system will have to be bled.

10. Tighten the bleed screw, close its protective cap and add new brake fluid to the reservoir.

11. In the same way, pump out the old brake fluid sequentially from the remaining calipers in the following order: front right, rear left, rear right.

Caution: The drained brake fluid must in any case be clean and free of air bubbles.

12. Adjust the brake fluid level according to the previously applied mark.

13. Press the brake pedal and check its free travel. Make sure that the brake pedal does not "sink" when you press it - the free travel of the pedal should be no more than 1/3 of its full travel.

14. Check the brakes on a road with light traffic. To do this, you should perform at least one strong braking, checking the ABS at the same time. A sign of proper ABS operation is pulsation of the brake pedal when braking.

Note: The effect of ABS is best demonstrated on unpaved roads.