Symptoms
- Pedal feels very firm and requires a lot of leg force to stop
- Braking may still work but feels much harder than normal
- Possible hissing sound when pressing the pedal
Likely causes
- CommonFailed brake boosterThe booster uses engine vacuum (or an electric pump) to multiply pedal force; when it fails, you're braking almost unassisted.
- CommonCracked or disconnected vacuum hose to the boosterA leak in this hose reduces the assist the booster can provide.
- PossibleFaulty check valve on the boosterThis one-way valve keeps vacuum in the booster; if it fails, assist drops.
- Less commonEngine running poorly, reducing available vacuumA rough-running or vacuum-leaking engine can indirectly reduce brake assist.
What to do
- Check if the pedal is hard from the very first press, which points to no vacuum assist at all.
- Listen for a hissing sound near the firewall when pressing the brake pedal, which suggests a vacuum leak.
- With the engine off, pump the brake pedal several times, then hold it down and start the engine; the pedal should sink slightly if the booster is working.
- Inspect the vacuum hose running from the engine to the booster for cracks or a loose connection.
- Drive cautiously and allow more stopping distance until this is repaired, since braking still works but requires much more effort.
- Have a mechanic test the booster and vacuum system, and replace the booster or hose as needed.
Tools and parts
Brake boosterSearch: “brake booster for your car make and model”
Brake booster vacuum hoseSearch: “brake booster vacuum hose for your car make and model”
When to call a pro
See a mechanic to test the booster and vacuum hose, since a fully failed booster makes emergency stops much harder.
Safety
Use caution
Braking still works but takes much more force and distance; leave extra following distance and get it checked soon rather than continuing normal driving.