Audi Cars 1999-05

Hydraulic System


Hydraulic System Bleeding

The clutch and brake hydraulics use an approved DOT 4 brake fluid in their systems. The hydraulic system should be bled periodically, usually a minimum of every two years, or more often under severe use. Brake fluid can absorb moisture. Because condensation is heavier than brake fluid, and under normal conditions, will not mix with it, the sediment and condensation tend to collect in the slave cylinder and in the bottom of the master cylinder reservoir.

If possible, before bleeding the system, use a siphon gun or a cooking baster to remove the old fluid, or remove the reservoir and flush it using a recommended brake cleaner or rubbing alcohol. If necessary, the reservoir can be flushed while removed from the vehicle using lightly pressurized water, however the reservoir must be flushed with brake cleaner or rubbing alcohol and dried completely before installation.

There are a number of brake hydraulic bleeders available that use vacuum to 'pull' the fluid through the hydraulic system. The advantage these tools offer is that the system can be bled without the help of an assistant.

Bleeding or flushing the clutch hydraulic system with dirty or contaminated fluid may result in component damage.

When bleeding a clutch hydraulic system, always observe to following guidelines:

NEVER reuse brake fluid
Always bleed the system using fresh fluid
Do NOT use fluid which has been left uncapped
Use a recommended fluid which is compatible with the existing fluid

  1. Fill the reservoir with fresh fluid.
  3. Fit a flare or box end wrench onto the slave cylinder bleeder screw.
  5. Attach a (clear if available) rubber tube to the slave cylinder bleed screw and suspend it into a suitable clear drain container partially filled with brake fluid.
  7. Open the bleeder screw and have an assistant press the clutch pedal to the floor.
  9. While holding the clutch pedal to the floor, close the bleed screw.
  11. Slowly release the clutch pedal to its fullest extension and recheck the reservoir fluid level. Top off as necessary.
  13. Repeat the above steps until the fluid exiting the bleed screw is clean and free of all air bubbles.
  15. Top off the clutch master cylinder reservoir as necessary.

Click image to see an enlarged view

Fig. A vacuum tester and a bleeding attachment (shown bleeding the brakes here) eliminate the need for a helper. These units are available from automotive vendors

The clutch system can be bled using a pressure bleeder. Follow the instructions that come with the pressure bleeder for the proper pressure bleeding procedure. The maximum line pressure while pressure bleeding must not exceed 36 psi (248 kPa).

  1. Before servicing the vehicle, refer to the precautions section.
  3. To bleed the system perform the following:
    1. Top off the hydraulic fluid reservoir using a fluid that meets the standards of the vehicle-s hydraulic system.
    3. Open the clutch slave cylinder bleed screw and press the clutch pedal to the floor and hold the pedal down.
    5. Close the clutch slave cylinder bleed screw.
    7. Release the clutch pedal.
    9. Check the hydraulic fluid level and top off as necessary.

  5. Repeat the above steps until the discharged fluid is clean and no air bubbles appear during the bleeding process.