Bad Brake Booster Symptoms: Hard Pedal, Hissing, Stalling and How It Is Tested

• Hard brake pedal: the pedal feels stiff and needs much more force, and the car takes longer to stop. This is the classic sign.
• Hissing sound when you press the pedal, from a vacuum leak at the booster or its hose.
• Engine stalls or idles rough when you press the brake, because a leaking booster draws extra air into the engine.
• Pedal that sinks slowly to the floor is more often a master cylinder or brake fluid leak problem than a booster fault.
• Safety: brakes are safety-critical. If the pedal is hard or the car does not stop normally, do not keep driving it; have it towed or inspected before driving. The brakes still work without power assist, but they take far more effort and distance.

A power brake booster sits between the brake pedal and the master cylinder, usually on the firewall on the driver side. On most gasoline cars it uses engine vacuum, through a hose and a one-way check valve, to multiply the force from your foot. If it fails, the hydraulic brakes still work, but they take much more pedal force. Diesels, many hybrids and electric vehicles use an electric vacuum pump or an electro-hydraulic booster instead, so symptoms and tests differ by vehicle. Check the owner’s manual or ask a technician.

1. Hard or stiff pedal. Pedal travel feels normal, but it feels like pressing a brick. Stopping distance increases.
2. Hissing at the pedal or under the hood. A leak in the booster diaphragm or vacuum hose can sound like air escaping when you press or release the brake.
3. Engine runs rough, idles high or stalls when braking. A big vacuum leak lets unmetered air into the intake. A check engine light with lean codes may appear.
4. Brake pedal height changes. The pedal may sit higher than usual or feel inconsistent.
5. Longer stopping distance without fluid loss or grinding noises.

Other causes of a hard pedal include a collapsed or leaking vacuum hose, a failed check valve, a stuck brake caliper, seized pistons or an engine with a vacuum problem. This is why a technician checks several parts, not just the booster. For grinding or shaking, see grinding noise when braking and brake pulsation.

This is a safe check that does not touch the brake hydraulics. It works on a vacuum booster. Do it on level ground, in park or neutral with the parking brake set.

1. With the engine off, press the brake pedal firmly four or five times until it feels hard and high. That uses up the stored vacuum.
2. Hold steady pressure on the pedal.
3. Start the engine while keeping your foot on the pedal.
4. A working booster lets the pedal drop slightly (about half an inch to an inch) as vacuum builds. If the pedal does not move, the booster, hose or check valve is probably not working.

A second check: with the engine running, turn it off while holding the pedal down. A good booster keeps the pedal firm for about 30 seconds; if it rises or turns hard, there is a leak. A hissing sound while you hold the pedal also points to a leak. Any result other than normal means the car should be inspected by a mechanic before driving. A shop can also test with a vacuum gauge and inspect the hose and check valve.

The two sit together, and symptoms overlap, but the typical difference is feel. A bad booster gives a hard pedal that does not sink. A bad master cylinder tends to give a soft, spongy or slowly sinking pedal, a brake warning light, or low brake fluid. Brake fluid leaking into the booster from the rear of the master cylinder can also damage the booster diaphragm, so a mechanic often checks both. Brake fluid loss, a warning light or a pedal that goes to the floor is an emergency: do not drive. Brake hydraulic repairs, bleeding and master cylinder work should be done by a qualified mechanic, not at home.

Costs vary by vehicle and region. A vacuum hose or check valve repair may cost roughly $50 to $250. Replacing a brake booster commonly runs from about $300 to $1,000 or more for parts and labor, and some vehicles, especially those with hydraulic or electric boosters, cost more because the master cylinder and cabin parts are involved. Remanufactured parts can lower the cost. Get a written estimate and ask the shop to show you the diagnosis, such as the leak or vacuum reading. Use our mechanic quote check if the number feels high.

Brake assist problems are sometimes the subject of safety recalls. Enter your VIN at NHTSA.gov/recalls to see open recalls, which are repaired free of charge by the dealer. Also see our guide to checking recall history by VIN.

• NHTSA: Vehicle Recalls (search by VIN)
• NHTSA: Vehicle Safety
• Car Care Council