Servo brake and Accessories for BMW Serie 3 - E30

How the power assistance is supplied

The brake booster requires a constant vacuum to amplify the force applied by the foot. Where this vacuum comes from depends on the engine.

• In a petrol engine, it is drawn directly from the intake manifold, where a vacuum naturally occurs at idle and under light load.
• On a diesel engine, the manifold does not produce a vacuum: an engine-driven vacuum pump takes care of this.
• In between, a non-return valve maintains the vacuum in the brake booster when the engine is no longer producing it, for example at full throttle or when the engine is switched off. It is this valve that allows you to make two or three assisted braking manoeuvres after the engine has stopped.
• The vacuum hose connects the whole system. It must be reinforced so as not to collapse under the vacuum, which an ordinary hose cannot withstand.

Stiff pedal: check for leaks before inspecting the component

A pedal that has become hard to press means that the power assistance is no longer coming through. This does not necessarily mean that the brake booster is faulty.

• The vacuum hose softens with age and collapses, or splits at its ends, where it is fitted. Squeeze it: it must remain firm.
• Ifthe non-return valve remains open, it allows the vacuum to escape. A simple test: with the engine switched off after running for a few minutes, you should still be able to apply the brakes with power assistance at least twice. If the first press already feels stiff, the valve or the hose is at fault.
• A whistling sound when braking and an increase in idle speed when the brake is applied indicate air ingress – i.e. a leak – rather than a punctured diaphragm.
• The vacuum pump on a diesel engine can lose efficiency without a clear fault: the power assistance then becomes insufficient when the engine is cold, before returning to normal.

It is only after ruling out these three causes that the brake booster itself becomes a suspect.

Replace the hose and the valve

The vacuum hose is not an ordinary hose: it is reinforced to resist crushing, and is often moulded with its bends and valve connection. A fuel hose fitted in its place will flatten during use and eliminate the power assistance in exactly the same way as a leak.

The non-return valve has a specific direction of fit: it allows air to flow from the brake booster to the manifold, and not the other way round. If fitted the wrong way round, it will immediately cut off all power assistance. Check the arrow or blow into it before fitting. Take the opportunity whilst replacing it to check the valve base seal on the brake booster, which also hardens over time, and to ensure that the hose is not rubbing against any hot components.

Before replacing the brake booster

If the vacuum circuit is in good working order and there is still a lack of power, the brake booster is at fault, most often due to its diaphragm. Another tell-tale sign: brake fluid in the brake booster, visible when the master cylinder is removed, indicates that the master cylinder’s seal is leaking and has damaged the diaphragm. Replacing the brake booster without addressing the master cylinder simply means you’ll have to do the job all over again.

To order a replacement, note down the diameter of the brake booster, the number of diaphragms (single or double), and the length of the push rod. Finally, check the condition of the underbody panel to which it is attached: this area is often corroded, and a new brake booster fitted to a worn underbody panel will not operate correctly.