Brake Boosters
A brake booster, or servo, multiplies the force applied to the brake pedal using engine vacuum, so that normal pedal effort produces enough hydraulic pressure to stop the vehicle.
What it does
A diaphragm inside the booster has engine vacuum on one side and atmospheric pressure on the other when the pedal is pressed. The pressure difference adds force to the pushrod that drives the master cylinder.
Signs a brake booster is failing
- A brake pedal that has become noticeably hard to press
- Much longer stopping distances with no fluid loss
- A hissing noise from behind the pedal when braking
- The engine idling roughly or stalling when the brake is applied
When to replace them
Not a scheduled item. Replaced on failure, which is usually a perished diaphragm or a failed check valve.
What to confirm before ordering
Have these ready and the counter can usually confirm the part in under a minute.
- Whether the fault is the booster or the vacuum supply, which is the more common cause
- Booster diameter and whether it is a single or tandem unit
- Master cylinder mounting pattern and pushrod length
- Whether the vehicle uses a vacuum pump rather than manifold vacuum
Brake Boosters questions
How do I test a brake booster?
With the engine off, pump the pedal several times to exhaust the stored vacuum, then hold the pedal down and start the engine. The pedal should sink slightly as vacuum builds. If it does not move, the booster or its vacuum supply is at fault.
Why does the engine stall when I brake?
Usually a ruptured booster diaphragm drawing unmetered air into the inlet manifold, which upsets the fuel mixture. It is a vacuum leak that only appears when the pedal is pressed.