Symptoms
- Audible hiss near the dashboard or firewall area when pressing the pedal
- Brake pedal may feel harder to press than usual
- Possible slightly rough idle
Likely causes
- CommonCracked or loose vacuum hose to the brake boosterA small crack or loose clamp lets vacuum escape, creating a hissing sound.
- CommonFailing internal diaphragm in the brake boosterA worn diaphragm can leak vacuum internally, both hissing and reducing assist.
- PossibleFaulty check valve on the boosterCan hiss as vacuum leaks past a worn seal.
- Less commonUnrelated vacuum leak elsewhere in the engine that happens to be more noticeable during brakingWorth ruling out if the hiss doesn't clearly trace to the booster area.
What to do
- Listen carefully to identify exactly where the hiss is coming from, ideally with the hood open and engine running.
- Check the vacuum hose connecting the engine to the brake booster for visible cracks or a loose clamp.
- Note if the pedal feels harder to press along with the hissing sound.
- Check for a rough idle, which can indicate a broader vacuum leak.
- Have a mechanic pinpoint the exact source with a smoke test or visual inspection if you can't find it yourself.
- Have the hose, booster check valve, or booster itself replaced depending on the source.
Tools and parts
Booster vacuum hoseSearch: “brake booster vacuum hose for your car make and model”
Brake boosterSearch: “brake booster for your car make and model”
When to call a pro
See a mechanic if you can't locate the source of the hiss yourself, or if the pedal feels harder to press than normal.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.