Description, removal and installation of the brake system vacuum hydraulic booster (BMW 5 Series E28)

Brake fluid is poisonous. It is also a strong paint solvent. See Warning at the beginning of Section Bleeding the hydraulic brake system.

Description

On E28 models (in the "retro" style) a vacuum hydraulic booster is used. The booster unit is located between the brake pedal (for left-hand drive models) or cross shaft lever (for right-hand drive models) and the master cylinder, is driven by fluid pressure produced by the power steering pump. When the engine is running, the steering pump supplies pressure to the pressure regulator/accumulator. The regulator/accumulator stores and regulates the pressure in the vacuum power steering. When the brake pedal is depressed, the pressure in the booster helps to operate the master cylinder, reducing the amount of pressure required to press the pedal.

The vacuum power steering is not subject to major repairs; if it fails, it should be replaced. Checking the system's operation requires special equipment, so even diagnosing faults is beyond the scope of a home mechanic. If the system fails, take it to a dealer service department or other auto repair shop.

Removal and installation

1. With the engine off, discharge the hydraulic accumulator by depressing the brake pedal 20 or more times.

2. Remove the master cylinder (see section Removal and installation the master brake cylinder).

3. Clean the area around the inlet and return line unions, then disconnect them. Plug the tubes to prevent dirt from entering the system and fluid from leaking.

Even the tiniest bit of dirt can damage your amplifier, so be especially careful not to let dirt into your system when the tubes are disconnected.

4. Working inside the passenger compartment, remove the lower left trim panels above the brake pedal (for left-hand drive models) or a storage box and trim (for right-hand drive models). On left-hand drive models, also disconnect the pedal return spring.

5. Remove the retaining clip and disconnect the push rod from the brake pedal or cross shaft arm.

6. Remove the four mounting nuts and remove the vacuum booster.

7. Installation is the reverse of the removal procedure. Tighten the hydraulic line unions to the required torque.

Do not attempt to tighten these connections without a wrench. If they are not tightened properly, they may leak, which will affect the operation of the system; if you overtighten them, you can damage the threads, which will also lead to leakage. To tighten these connections correctly, you will need a special split socket on your wrench.

8. After finishing the work, bleed the hydraulic brake system (Section Bleeding the hydraulic brake system) and adjust the brake pedal travel and brake light switch (see section Adjusting the brake pedal).