12.9. Description, removal and installation of the vacuum hydraulic booster of the brake system

Brake fluid is poisonous. It also strongly dissolves paint. See the Prevention at the beginning of the Section Pumping of Hydraulic System of Brakes.


On the E28 models (in retro style) the vacuum hydraulic booster is used. The block of the amplifier located between a brake pedal (for models with left-side management) or the lever of a cross shaft (for models with right-hand management) and the main cylinder, is put in action by the liquid pressure made by the steering pump with hydrostrengthening. At the switched-on engine the steering pump gives pressure upon pressure regulator/accumulator. The regulator/accumulator accumulates and regulates pressure in the vacuum hydraulic booster. When pressing a brake pedal pressure in the amplifier helps to put the main cylinder in action, reducing necessary effort of pressure upon a pedal.

The vacuum hydraulic booster is not subject to capital repairs; in case of refusal it should be replaced. Check of work of system demands special devices therefore even diagnostics of malfunctions is not included into competence of the house mechanic. If the system refuses, give it to dealer office of service or other automobile repair shop.

Removal and installation


  1. At the switched-off engine discharge the hydraulic accumulator, having squeezed out a pedal of a brake of 20 or more times.
  2. Remove the main cylinder (see the Section Removal and installation of the main brake cylinder).
  3. Clear area around nipple connections of inlet and returnable lines, then disunite them. Stop up tubes to prevent hit of dirt in system and leak of liquid.

Even the tiny part of dirt can damage the amplifier therefore be especially careful, do not allow hit of dirt in system at the disconnected tubes.

  1. Working in passenger office, remove the lower left panels of a covering over a brake pedal (for models with left-side management) or a ware box and a covering (for models with right-hand management). On models with left-side management also disconnect a returnable spring of a pedal.
  2. Take a fixing clip and disconnect a pusher from a pedal of a brake or the lever of a cross shaft.
  3. Give four fixing nuts and remove the vacuum amplifier.
  4. Installation repeats the procedure of removal upside-down. Delay nipple connections of hydraulic lines with the required effort.

Do not torture to delay these connections without the aid of a key. If it is bad to tighten them, then they can escape that will influence work of system; if to draw them, then it is possible to damage a carving that will also lead to leak. For the correct delay in these connections you will need a special cutting cap nozzle on your wrench.

  1. Upon termination of work pump over hydraulic system of brakes (Section Pumping of Hydraulic System of Brakes) and adjust the course of a pedal of a brake and the switch of a stoplight (see the Section Adjustment of a Pedal of a Brake).