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Your Brake booster (vacuum servo booster) worn out? » Read more about common malfunctions • Repair • Replacement manual
Braking with air pressure support: the brake booster
The brake booster makes use of pneumatic principles, enhancing the power exercised on the pedal during braking. The normal air pressure is used to generate a vacuum. Replacement of the brake booster requires some work, though it is not very complicated in the technical sense.
The trick with the vacuum
To understand how the brake booster works, a simple trick helps: when the car is rolling downhill with the engine switched off, the brake booster is disabled. Braking is considerably heavier as the pedal builds up a higher resistance. The brake booster uses the normal air pressure to increase the pressure on the main brake cylinder (master cylinder) by generating a vacuum. Stepping on the brake pedal causes the air pressure to press on the master cylinder. The underlying technology is quite complex, though has been successfully applied for several decennia.
Brake booster: position and construction
The brake booster is fixed to the bulkhead between engine and the car interior, located on the driver's side. The component is very easy to spot. It is the big, black box with the master cylinder screwed onto it. The brake booster is connected to the brake pedal or its deflection mechanism, which extends into the passenger cabin. An essential part of the brake booster is maintenance of the vacuum. The large vacuum box must permanently be sucked empty to maintain the vacuum. Several techniques are at work here. Usually the engine is used to generate the vacuum. Sometimes, separate pumps take on this task. They are generally driven by a belt drive from the engine. Cars with diesel engines often have separate pumps as this engine type is not optimally suitable for generating vacuum.
Brake booster defects
A defective brake booster is noticed very soon. The counterforce of the brake pedal increases suddenly. Stopping the car is not immediately necessary, although it should now be driven slowly in the direction of the nearest garage. The brake function is not influenced by the failing brake booster, still the driver has to apply a lot more force to attain the desired effect. Causes for a defective brake booster can be:
- the extraction (suction) has become loose
- the extraction pump is defective
- clogged interior filter
- corroded casing
- defective internal membrane
- malfunction of the valves and deflection lines.
Symptoms can be a whistling and squeaking sound, noticed during braking. The loss of pressure is also clearly felt.
Repairing the brake booster
Repair of the brake booster is a lot of work and not really an option for technical laymen. The component consists of several separate components which are welded or glued together. In case of emergency, a corrosion hole can be welded in the casing. However this is not a permanent solution. The real solution is replacement of the entire component by a new brake booster. This requires some work which is relatively easy. The brake booster is fixed to the bulkhead with two bolts. On its other side it is attached to the master cylinder also with two bolts. A mechanical connection exists between both these components. The brake lines connecting the master cylinder are rigid and therefore components must be removed together. The best course of action is: 1. draining brake fluid 2. disconnecting the brake pedal from the linkage 3. disconnecting the brake lines from the master cylinder 4. unscrewing the brake booster 5. removing both brake booster and master cylinder 6. separating master brake cylinder and brake booster after their joint removal. This procedure saves a lot of tricky work and is also a perfect occasion for a master cylinder overhaul and a brake fluid change. Costs for a repair set for the master brake cylinder start at 10 EUR (± £9), and therefore this occasion should be made use of. It doesn't get easier than this. With a freshly overhauled master cylinder the brake fluid change poses no problem at all. Mind the right order: changing the brake fluid could cause the master cylinder to damage the internal gaskets. Therefore: first the overhaul or replacement of the master brake cylinder and then renewal of the brake fluid.
Costs for a new brake booster
A high-quality brake booster of branded quality (e.g. ATE) costs ca. 100 EUR (± £90).