Audi Cars 1999-05

Power Brake Booster


Removal & Installation

The brake booster cannot be serviced. If failed or damaged, it must be replaced.

If the vehicle is equipped with the Electronic Stability Program (ESP), the ESP pump must be actuated during the bleeding procedure, using the VAG 1551, VAG 1552, or equivalent scan tool, capable of activating the ESP pump.

  1. Disconnect the negative battery cable.
  3. Disconnect the electrical connector for the brake fluid reservoir level sender.

Bleeding or flushing the brakes with contaminated fluid could damage the master cylinder.

  1. If the fluid is discolored and/or contaminated with debris, remove and flush the reservoir using an approved brake cleaner. If the fluid is in relatively clear and in good condition, open the bleeder on the left front caliper and bleed out all of the fluid from the brake master cylinder and close the bleeder.
  3. If equipped, disconnect the clutch hydraulic hose.
  5. Cover the engine compartment to catch any residual brake fluid.
  7. If the brake master cylinder reservoir spigots enter the side of the master cylinder bore, the reservoir is attached to the bottom of the master cylinder. Release the reservoir by pressing down on the locking catch and pulling the reservoir out of the master cylinder sealing plugs.
  9. Disconnect and plug all the brake lines from the master cylinder.
  11. Remove the instrument panel trim from the bottom/left inside the passenger compartment.
  13. Remove the brake light switch.
  15. Use a long sturdy hook tool, such as Tool No. T10006 brake pedal release tool. Place the tool just below the brake servo actuating rod ball socket. Pre-tension the brake pedal upward by pulling it up using your toes, then gently pull up on the release tool to disconnect the brake pedal from the with the actuating rod.
  17. Remove the 2 T-45 Torx® bolts from the center of the master cylinder mounting flange nuts, then pull the booster and master cylinder out of the plenum chamber. Make sure to clean up any spilled brake fluid immediately. If necessary, dilute any spilled brake fluid with water.
  19. Separate the master cylinder from the booster assembly.

To install:

  1. Installation is in reverse order of removal. Note the following:
    1. Measure from the flat mating surface to the ball head of the vacuum servo unit actuating rod. Make sure the rod is at a right angle to the mating surface. The distance must be 6.240-6.280 inches (158.5-159.5mm) to the flat mating surface WITHOUT a gasket.
    3. Torque the following items to specification indicated:

      Brake booster-to-plenum chamber (T45 Torx®): 18 ft. lbs. (25 Nm)
      Master cylinder-to-booster (replace): 37 ft. lbs. (50 Nm)

    5. Connect the reservoir and lines. Connect the brake pedal to the pushrod. Adjust the brake light switch. Fill and bleed the brake system.
    7. Once completed, pump and check the brake pedal for a firm feel, then, if the pedal pressure is firm, test drive the vehicle to be sure the brakes are operating correctly and that the pedal feel is firm during braking.

If the brake pedal has good pedal pressure after bleeding, but later requires being pumped up to regain pedal pressure, check the system for fluid leaks. If no fluid leaks are present, the master cylinder has most likely failed.


System Operation
  1. With the engine OFF , pump the brake pedal until the supply vacuum is entirely gone.
  3. Put light, steady pressure on the brake pedal.
  5. Start the engine and let it idle. If the system is operating correctly, the brake pedal should fall toward the floor if the constant pressure is maintained.

Power brake systems may be tested for hydraulic leaks just as ordinary systems are tested.

Clean, high quality brake fluid is essential to the safe and proper operation of the brake system. You should always buy the highest quality brake fluid that is available. If the brake fluid becomes contaminated, drain and flush the system, then refill the master cylinder with new fluid. NEVER reuse any brake fluid. Any brake fluid that is removed from the system should be discarded.

Vacuum Leak Test
  1. Operate the engine at idle without touching the brake pedal for at least one minute.
  3. Turn off the engine and wait one minute.
  5. Test for the presence of assist vacuum by depressing the brake pedal and releasing it several times. If vacuum is present in the system, light application will produce less and less pedal travel. If there is no vacuum, air is leaking into the system.