The brake booster is part of the brake assistance system in modern vehicles. Its job is to amplify the pressure on the brake fluid when you press the brake pedal. This makes braking easier by reducing the effort needed to brake the car effectively. So this means that if the brake booster is failing then you should notice a drastic difference in how the brakes perform.
The symptoms of a bad brake booster include brakes that don’t stop the car as effectively as before, longer stopping distances, a harder brake pedal and sometimes an engine that cuts out when you brake hard.
The brake booster works by creating a pressure vacuum in the brake lines that significantly increases the pressure applied to the brake pads when you push in the brake pedal. There are various types of brake boosters fitted to todays vehicles, with the most popular being the dual-piston master cylinder.
Brake boosters usually fail if because of air getting into the system and this prevents a strong enough vacuum from being produced.
If you suspect there is something wrong with the brake booster (also called the vacuum servo) on your car, it’s very important to get it checked out by a mechanic right away.
5 Common Symptoms Of A Bad Brake Booster
Generally, 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.
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.
If you are driving a petrol or diesel car, then the symptoms will be the same no matter your car model, year, or manufacturer.
Brake boosters can fail in any car, and it’s usually caused by a vacuum leak.
Below is a list of the most common bad brake booster signs.
Symptom 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 sign of a bad brake booster. It is also one of the most problematic and dangerous. A “harder brake pedal” is a subjective term.
A hard brake pedal is a classic symptom of a bad brake booster
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.
Symptom 2. The Brake Pedal Is Higher Than Usual
Another typical bad brake booster symptom 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.
Symptom 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.
Symptom 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.
Symptom 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.
A diagnostic code reader can be used to confirm a bad brake booster
Most vehicles also have a dedicated Brake Booster Pressure Sensor that immediately informs the ECU about any problem with the vacuum.
Any problem with engine vacuum pressure, or brake booster pressure can also trigger the check engine light and/or the brake warning light.
What Next – How to check for a bad brake booster yourself at home
The brake booster on most cars should be easy to access and depending on the type of vehicle you may be able to check the booster and connected hoses for problems. If the brakes are not working at all or if there is brake fluid leaking from the brake system then you should not drive the vehicle and you should get it checked out by a qualified mechanic.
Here is a video that explains in great detail how the brake booster works and how to test the brake booster seals and diaphragms or connected hoses and valves for faults.