Checking the performance of the vacuum booster (BMW 3 Series E21)

Press the brake pedal ten times with the engine off to create equal pressure in the vacuum booster cavities, close to atmospheric.

Keeping the brake pedal depressed, start the engine. If the vacuum booster is working properly, the brake pedal should "go forward" after the engine starts.

If the pedal does not "go forward", check the condition and fastening of the check valve and vacuum hose, as well as the condition of the rubber ring that ensures the tightness of the connection between the vacuum booster and the master cylinder.

If there are no defects, connect a vacuum gauge between the vacuum booster and the check valve and measure the vacuum coming from the intake manifold, which, when the warmed-up engine is idling, should be at least 40 kg/cm².

[The original article is located on BMWMAN]