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Bad Brake Booster Symptoms and Replacement Cost

Brake boosters are to brakes as power steering is to steering. The goal of a brake booster is to reduce the amount of force the driver needs to exert when applying the brakes. They are also referred to as servo vacuums or vacuum brake boosters because they use a stored vacuum system in order to overcome high fluid pressure.

In order to determine if your brake boosters are bad we first need to look at how they work.

How Do Brake Boosters Work?

The brake booster is situated between the brake pedal and master cylinder. As the driver begins to press down the on brake pedal, the brake booster amplifies the force being exerted on the master cylinder. As a result the driver doesn't have to push down very hard in order to engage the braking system.

The brake booster itself is a canister made up of two chambers separated by a diaphragm with a rod that runs through the center of it. The rod connects the brake pedal to the master cylinder.

hydraulic disc brake system diagram

When the brake pedal isn't engaged, the chambers create a vacuum. As you press down on the brakes, air enters into one chamber as the other maintains the vacuum. The air pressure in the other chamber increases and that exerts pressure on the diaphragm. This exerts a force on the rod connected to the master cylinder. As you take your foot off the pedal, the original vacuum state is achieved in both chambers.

As you can imagine, if the brake boosters are faulty, then braking becomes quite difficult. Let's take a look at some of the symptoms of bad brake boosters.

Symptoms of a Bad Brake Booster

1. Stiff Brakes

The first warning sign of a failing brake booster is when you find the brake pedal extremely hard to engage. Since the main function of the booster is to facilitate and ease the task of pushing the pedal, a faulty brake booster will make this process quite difficult.

This is because of excess pressure inside the master cylinder. This can also result in the brake pedal being higher than normal.

2. Spongy Brakes or Longer Stop Distance

As the problem with the brake booster check valve becomes worse the brakes can feel spongy or won't work at all, the pedal may even sink to the floor.

This happens over time as air bubbles start to move down the brake lines and into the brakes themselves. This causes a reduction of pressure in the brake lines causing them to be soft. While they may feel spongy, this situation is problematic as the brakes will also take longer to stop the vehicle.

In this case it's best to visit a mechanic. Brakes are a hydraulic system and in order for the air to leave they will need to be bled.

3. Engine Stalling

As we mentioned, the brake booster has a diaphragm between the two chambers. The diaphragm prevents air from getting through the seal. When the brake booster fails, it can draw excess vacuum from the engine.

As a result when the brakes are engaged the engine will feel like it is stalling and the idle can drop.

Troubleshooting and Testing the Booster

To ensure the brake booster is the problem you can test this problem at home.

  1. Turn the engine off.
  2. Pump the brakes 5-6 times. This will get rid of the residual vacuum.
  3. Turn the engine on while pushing lightly on the brake pedal.
    1. If the brake booster is working normally it will go down a little and then become firm.
    2. If the brake booster isn't working correctly, nothing will happen. That or you'll notice the first symptom (stiff pedal).

The video below by Educational Mechanics demonstrates this process:

Booster Replacement Cost

The replacement cost of a brake booster largely depends on the make and model of your vehicle.

Although, generally, the amount ranges from $300 to $700. However, the price may go up or down depending on the labor cost, parts, fees, and taxes in your area.

Final Thoughts

The brake booster is an important aspect in the performance, as well as the safety of your vehicle. If you notice that your brake pedal is stiff, the brakes feel spongy, or you begin to idle when you brake, then your brake booster might be the problem.

Either way, we strongly recommend you to consult a mechanic if you suspect a problem with your brakes. Doing so will prevent potential accidents and keep you and other drivers safe on the road.


Richard Reed
I've been a General Manager of a moving company and I've also been a Professional Mover for over 30 years. I've driven flat beds, reefers, dropdecks, moving vans, heavy machinery, etc. In my time as a Mover I've driven over 1,000,000 safe miles. My days of moving and driving truck are past me but The Vehicle Lab allows me to share the knowledge I've gained over the past 40 years.

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