Top Bad Brake Booster Symptoms

It’s rare for a brake booster to fail, but when it does, it’s easy to notice. When you have to press the brake pedal really hard in order to slow down, that means you probably have a bad brake booster.

“You do not need to panic, though— just keep pressing harder,” says Chris “Moose” Pyle, an auto expert with JustAnswer.com. “The rest of the brake system is working. You can still drive, but you will get really tired, and you will need to start your braking sooner, which means you need to get this addressed sooner rather than later.”

Often times, brake booster issues can be traced back to other problems within your braking system, such as worn brake pads or a brake fluid or vacuum leak. If you are concerned you might have a bad brake booster, here’s what to know.

The Important Role of the Brake Booster

The brake booster plays an important role in stopping a car equipped with disc brakes. Located under the hood, the device delivers additional force to the car brakes. Without it, stopping distances increase dramatically.

“Identifying problems with your brake booster is crucial to maintaining safety while on the road,” says Duane “Doc” Watson, a technical trainer at Bosch Mobility Aftermarket. “Lack of a well-functioning brake booster compromises your ability to operate the vehicle as you normally would, which puts you at greater risk of losing control and getting into an accident.”

Two chambers inside the brake booster, separated by a diaphragm, experience a similar drop in pressure as air rushes into the engine through the air intake and creates a vacuum. Engaging the brake pedal opens a valve, and air is drawn into one side of the booster. This allows the vacuum, present on the other side, to pull on the diaphragm. A rod attached to the brake pedal assembly runs through the center of the diaphragm on its way to the master cylinder piston.

As the rod transfers the force applied to the brake pedal, the diaphragm also pulls on the rod and increases the force against the master cylinder piston. The combined force slows the car quickly and easily. By releasing the brake pedal, the valve closes, and the system returns to balance.

If you recognize the following bad brake booster symptoms, take action immediately — don’t ignore them. They indicate that your car has become unsafe to drive.

Stiff Brake Pedal Action

A bad brake booster makes the brake pedal much harder to depress. As the booster fails, it loses its ability to provide additional force against the master cylinder piston. The driver of the vehicle must now provide all of the braking power — a difficult task.

Increased Braking Distance

Sometimes a brake booster will fail gradually, making the decline in performance harder to notice. If you feel like you must brake earlier than you should to come to a stop, have the brakes checked. If the pads and rotors look good, the cause could be the brake booster.

High Brake Pedal Position

Along with the stiffer resistance of the brake pedal under pressure, you may also notice the pedal “travels” less than normal when the brakes are engaged. With the booster chambers out of balance, pedal return may be slower than normal.

Poor Engine Function

A hole in the diaphragm allows the booster to draw air into the induction system. This situation can effect the fuel mix in the engine because of the booster’s connection to the main air intake. In newer, fuel-injected vehicles, this results in reduced fuel economy because the computer compensates for the difference. In older cars, brake booster failure may cause the engine to run lean with insufficient fuel in the cylinder.

Without the full cooling action of the proper measure of gas, friction increases, and the temperature of engine parts rises, leading to pre-ignition (the fuel ignites before the spark plug fires). This might be heard as pinging, or felt as a loss of engine power. Allowed to continue, pre-ignition can lead to burnt valves or piston damage, requiring a costly repair.

Bad Brake Booster Symptoms

The most obvious sign of a bad brake booster is if you have to use extra force on the brake pedal.

“You’ll likely have to muster a great deal more strength than usual to force your vehicle to slow down, and even then, it will not do so as quickly or efficiently as it normally would,” says Watson.

The check engine light on your dash might also illuminate. If the source of the problem is a vacuum leak, you might hear a whistling sound or feel a stumbling sensation when you brake, he says. The latter stems from the engine, which could be losing vacuum from its other components in the process.

What To Do If Your Brake Booster Fails

If you notice your brakes showing greater resistance to the point you are having difficulty stopping, immediately switch your turn signal on, pull to the side of the road and light up your hazards, says Watson. If you think you can control the brakes enough to make it to a nearby repair shop, allow yourself plenty of stopping distance between your car and the vehicle driving in front of you.

“The bottom line is, you need to get your car into a repair shop as soon as possible,” he says. “Continuing to drive with brakes that are unreliable in any way is highly dangerous, and the problem could get worse if you ignore it, potentially leaving you completely incapable of stopping.”

Test Your Brake Booster

Starting with the engine off, pump the brakes several times. (Brake booster design allows the mechanism to hold enough vacuum to engage the brakes once or twice without the engine running. This helps you stop the vehicle in the case of engine failure.) Next, press on the brake pedal lightly while starting the engine. You should feel the pedal give a bit and then firm up. If it becomes hard or if you feel no change at all, then the brake booster has failed.

About the Experts

• Duane “Doc” Watson is a technical trainer at Bosch Mobility Aftermarket. He has more than 45 years of experience in automotive repair, has trained thousands of technicians and earned many industry awards, including Chevrolet Technician of the Year and Buick Service Master of the Year.
• Chris “Moose” Pyle is a master-certified technician with 20-plus years of automotive experience. He has also worked as an expert for JustAnswer.com since 2006, where he answers hundreds of car maintenance and repair questions a day.