Troubleshoot Wheels

Symptoms of A Bad Brake Booster

bad brake booster symptoms
Matt Taylor
Written by Matt Taylor

If the brake booster on your car is faulty then the brake pedal will be harder to push down on. The brakes won't work as well as they should and it will take longer to stop.

If you suspect there is something wrong with the brake booster on your car, it's obviously very important to get it checked out by a mechanic right away.

In this article, I explain what a brake booster is and some common symptoms when they fail.

What Is A Brake Booster?

Most DIY mechanics know, that the brake system on pretty much every vehicle is a sealed hydraulic system. It works by compressing brake fluid that pushes the brake pads against a brake disc.

What many don't know, is that disc brakes introduced the necessity of assisting the driver when applying force to the pedal. This is due to the huge hydraulic pressure required to effectively squeeze the pads against the brake discs. 

brake pads and discs

It can take a lot of force to effectively apply pressure to brake discs and pads, especially in high performance vehicles

This hydraulic assistance is achieved by a vacuum operated device called a brake booster. 

If you try driving an old car that relies on brake shoes and cables to stop, you'll appreciate the assistance that the brake booster supplies!

In this article, we will review the symptoms of a bad brake booster and what to do when this vital part of the brake system is not working as it should.

What’s The Purpose Of The Brake Booster

The brake booster plays a crucial role in the braking system. 

As mentioned above, the brake booster helps the driver to overcome the enormous hydraulic pressure needed to apply the brakes, especially disc brakes.

symptoms of a bad brake booster

The brake booster (the black part above) boosts the effect of pushing on the brake pedal (Paul Day - Wikimedia Commons)

You can think of the brake booster as a sort of amplifier. It's usually located in the engine bay, under the windshield.

It uses the vacuum generated by the engine to help increase the pressure of the brake fluid in the brake lines.

This significantly reduces the amount of effort needed by the driver, and enhances the braking effect of the brake discs and pads.

Symptoms Of A Bad Brake Booster

Unlike many other car problems, identifying a problem with the brake booster is fairly straightforward. Most of the time you'll become aware of the problem because of a change in how the brake pedal feels and works.

bad brake booster

A typical brake booster (servo) from a car (Ildar Sagdejev - Wikimedia Commons)

If you are driving a petrol or diesel car, then the symptoms will be the same no matter your car model, year, or manufacturer.

Below is a list of the most common symptoms of a bad brake booster:

1. Harder brake pedal 

As you may guess, if the main function of the brake booster is reducing the force required by the driver, then when it starts failing you will notice a much harder brake pedal.

This is the most evident and widely known symptom of a bad brake booster. It is also one of the most problematic and dangerous. A “harder brake pedal” is a subjective term. 

Internally, the brake booster has a diaphragm, and when this diaphragm fails its usually caused by a small leak. Small vacuum leaks will drop brake booster efficiency and thus will require more energy from the driver.

If this goes unfixed, the diaphragm may completely fail, meaning zero servo assistance to the driver and a much longer braking distance. 

Just to give you an idea of the assistance you normally get from the brake booster, try this simple experiment.

Get into your car or truck, put the transmission in neutral and push the car forward (make sure it's safe to do so) and then try using the brakes. The pedal won't be as soft as usual, instead it should feel very hard. You will feel the amount of pressure you need to stop the car. Now imagine you are driving at 50 Km/h. A huge amount of force on the brake pedal would be needed to stop the vehicle. 

My recommendation is that if you start feeling a hard brake pedal then stop using the car and get it checked out as soon as possible.

2. The Brake Pedal Is Higher than Usual 

Another typical symptom of a bad brake booster is a change in the brake pedal height. You will see that the brake pedal is noticeably higher than usual when unpressed. 

The increased height will force you to lift your foot higher than usual thus creating another safety risk. Brake pedal height is designed to allow you to comfortably move your feet from one pedal to another.

The new brake pedal height is not only inconvenient but will consume more time to reach it. An extra second is sometimes the difference between braking in time or having a collision. 

3. Longer stopping distances

Even if you are an experienced driver and think you can apply enough force to the brake pedal truth is you won’t be able to stop the car in the same distance.

A hard brake pedal coupled with a higher pedal height will result in an increased amount of time to reach the pedal and then an increased amount of time to apply brakes hard enough to provide the master cylinder with the pressure required to stop the car.

It’s a matter of physics, in average your car may need a 20%-30% longer distance to stop. That figure assumes you are aware of the problem because if the brake booster fails while you apply the brakes it could take more than double the usual distance to stop the car due to the increased reaction time.

The worst-case scenario would be driving with a bad brake booster under adverse weather conditions like heavy rain or snow. 

4. Engine stalls when brakes are applied 

The three previous symptoms were closely related and are unquestionably typical of a bad brake booster.

While it is true that this symptom is also common of a bad brake booster, it is also true that could be related to other engine problems.

That said if your engine stalls when brakes are applied there is a high probability of a vacuum leak in the brake booster and/or its vacuum hose.

The amount of vacuum required by the brake booster is higher than any other engine accessory. That’s why a broken vacuum hose or a faulty brake booster diaphragm will cause major problems to the engine during idling.

In most cases, the Electronic Control Unit (ECU) won’t be able to stabilize the idle while applying the brakes. This symptom may not pose a safety threat like the others but is unquestionably annoying.

5. Vacuum Related OBD-II Engine Error Codes 

Modern ECU programming subroutines can effectively detect vacuum problems in the engine.

This is possible thanks to the Manifold Absolute Pressure sensor (MAP), and the brake pedal sensor. When you apply the brakes the computer knows it, and hence if the MAP sensor indicates a sudden drop in vacuum the ECU stores the corresponding error code in memory.

Some vehicles also have a dedicated Brake Booster Pressure Sensor that immediately informs the ECU about any problem with the vacuum.

What Causes A Brake Booster To Fail?

If you suspect the brake booster is failing then the vast majority of the time this will be caused by a vacuum leak.

The leak can either be in the brake booster itself, a leak in the hose that connects to the intake manifold or it can be an engine vacuum leak.

Most of the time the the problem will be caused by the hardening of the internal diaphragm of the brake booster or a problem with the way the brake booster connects to the intake manifold

Over time, hoses can become hard and brittle, and even a tiny crack in the rubber will cause a decrease in pressure. 

The internal diaphragm that separates the brake booster into two chambers is less likely to harden, especially in newer vehicles. However, it's not unusual for it to develop a leak or a fault over time through normal usage. 

How To Check Brake Booster Vacuum

Now that you know a little more about the brake booster, you've probably realised that most of the symptoms of a bad brake booster are vacuum related.

Fortunately, there is a simple way to check if the brake booster vacuum is causing the problem.

  1. Locate the brake booster and disconnect its vacuum hose from the engine side.
  2. Using an appropriate cap, close off the engine vacuum side of the hose (or plug the inlet to the intake manifold)
  3. Start the engine and check the idle. If the idle is normal then there is nothing wrong with the engine vacuum, and you may have a vacuum leak from the hose or the brake booster itself.

Another really easy way to check the brake booster is by pumping the brake pedal with the engine off. As soon as you start the engine, the brake pedal should go a little soft and become easier to push down on.

If this isn't the case after you start the engine, then there is something wrong with the brake booster, or the brake system.

How Do You Repair A Failed Brake Booster

Brake boosters are not easily repaired. If you are going to attempt a repair yourself then I'd strongly advise replacing the entire booster with a new or properly refurbished unit.

Replacing a brake booster is not the most difficult job. Usually it can be done without having to drain the brake system of fluid.

If you are going to attempt to replace the brake booster, then be sure to reference a repair manual for your exact vehicle. 

Here's a quick overview of the replacement steps that you will carry out on most vehicles:

  1. Locate the brake booster (it's normally up under the windshield).
  2. Remove and plastic covers, or other engine parts such as air filter boxes and hoses so that you can access the brake booster.
  3. Disconnect the vacuum hose (if visible) from the brake booster. Sometimes the hose is not removable, and you may have to disconnect the hose from the other end and replace the hose too.
  4. Next, move the brake fluid reservoir and disconnect the master cylinder. You should be able to do this without disturbing the brake fluid or brake lines, if not you'll need to drain the system.
  5. Now you should be able to access the brake booster and remove it from the vehicle. This procedure will be vehicle specific, so you'll need to use a repair manual for your vehicle.
  6. Installation of the brake booster is just the reverse of removal.
  7. When complete, be sure to carry out a full check and test, of the brakes before driving the vehicle.

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