Symptoms
- Brake pedal suddenly requires much more leg force than usual
- Possible hissing sound near the firewall when braking
- Engine may idle roughly if the booster has an internal vacuum leak
Likely causes
- CommonFailed diaphragm inside the brake boosterThe internal diaphragm can develop a leak, reducing or eliminating vacuum assist.
- CommonCracked or disconnected vacuum hoseInterrupts the vacuum supply the booster needs to work.
- PossibleFaulty check valve on the boosterLets vacuum bleed out of the booster instead of holding it.
- Less commonEngine vacuum leak elsewhere reducing available vacuum overallCan indirectly starve the booster even if it's mechanically fine.
What to do
- Check if the pedal is hard from the very first press or only after several presses.
- Listen for hissing near the firewall (where the booster connects to the engine) while braking.
- With the engine off, pump the brake pedal a few times, hold it down, then start the engine; the pedal should sink slightly if the booster works.
- Check for a rough or unusual idle, which might point to a vacuum leak through the booster.
- Drive cautiously and allow extra stopping distance until this is repaired.
- Have a mechanic test the booster and vacuum hose, replacing whichever is at fault.
Tools and parts
Brake boosterSearch: “brake booster for your car make and model”
Booster vacuum hoseSearch: “brake booster vacuum hose for your car make and model”
When to call a pro
See a mechanic to test and replace the booster or vacuum hose, since this affects how quickly you can stop in an emergency.
Safety
Use caution
A failed booster makes emergency stops much harder since braking still works but with far less assist; get it repaired soon and drive cautiously in the meantime.