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Why Is Brake Booster Failure Caused By Brake Fluid? What You Need To Know!

The brake booster enhances the performance of your brakes and ensures your car stops cleanly when you brake.


If you didn’t have a brake booster in your car, you’d have to push a whole lot harder on your brake to apply the same amount of force to the brake master cylinder making the brake booster an irreplaceable part of any classic car restoration build.


But what happens when a brake booster fails? What causes brake booster failure, and how can you recognize when your classic car’s brake booster is failing?


Find out what you need to know about vacuum brake booster failure below and how you can help prevent your classic car’s brake booster from failing.



How does a vacuum brake booster work?

In the vacuum brake booster, the internal diaphragm separates the two different sides.


These compartments are referred to as:

  1. The working chamber

  2. Vacuum chamber


As you release the brakes, there is an equal amount of vacuum on both sides. However, when you apply the brake pedal, a control valve allows more pressure into the working chamber.


The result of this operation is the activation of a pushrod that applies force to the master cylinder, which allows brake assist to aid your braking.

What are the symptoms of a faulty brake booster?

You should always be aware of the integrity of your brake boosters to ensure they’re working perfectly for you and your classic, muscle, or sports car.


By understanding the symptoms of a faulty brake booster, you’ll be able to figure out when they need to be maintained, repaired, or replaced before any significant damage is caused to your classic car’s engine.


Here are five symptoms of a faulty brake booster:

  1. Stiff brake pedal If your brake booster has lost the ability to amplify the force of your foot to the brake, then you’ll notice your brake pedal is a lot stiffer and requires more pressure to use.

  2. High brake pedal position When there’s an imbalance of pressure in the vacuum chamber, the pedal will sit higher than usual. It’ll also return slowly to the original position when you let go.

  3. Increased braking distance Not only is increased braking distance dangerous, but it’s also a clear indication of a bad or failing brake booster. Get your brake booster checked out immediately if you notice an increase in your braking distance.

  4. You can hear a hissing sound If you hear a hissing noise underneath the chassis or at the brake system, then this is a clear sign that your brake booster is damaged. This is usually caused when the suction begins to leak, which causes a crackling noise. If you hear this noise, then your braking system booster is faulty, or it’s about to get damaged. Every time you push your foot to the brake pedal and hear a noise coming from it, you’re damaging it further, so it's best to repair it as soon as you can. Note: If the sound is coming from your engine compartment or the brake sections, then it’s probably a failing brake booster.

  5. Engine is coughing If your engine is coughing and sputtering when you press down on the brake pedal, then you more than likely have a vac leak in the brake booster system.

What is the leading cause of brake booster failure?

Did you know that one of the leading causes of brake booster failure is brake fluid leaking into the brake booster?


When this happens, the brake fluid deteriorates the rubber diaphragms, seals, and valves, and eventually, the brake fluid eats any rubber components it comes in contact with.


As soon as the master cylinder seal gives out, the booster vacuum system will start to pull the fluid passed the seals and the rods in the booster. Then it will start to destroy the diaphragms as soon as the fluid enters the chamber.

What does a brake booster damaged by brake fluid feel like?

Most contaminated boosters will show signs of a soft pedal due to actually having a hydraulic leak. When a sealed system, such as a brake system, gets a leak, it won’t be able to build pressure. The fluid will just leak on and create a loose or soft pedal feel.


Depending on the damage from the brake fluid, you may also experience a hard brake pedal. For instance, if there is only damage to a diaphragm, the vacuum would hold steady inside the booster, creating hard pressure.


If you notice any change in the feeling of your pedal, then you might have a problem with your brake booster, and it would be best to get it checked out by a professional.

How to fix a brake booster damaged by brake fluid

As soon as brake fluid has contaminated the interior of the classic car brake booster, the booster must be taken apart immediately. All fluid has to be removed, and the booster needs to be cleaned. All seals, diaphragms, and other rubber parts must also be replaced.


Be sure to always rebuild or replace the master cylinder after you have your booster rebuilt, and be sure to bench bleed or prep your master cylinder as instructed by the master cylinder manufacturer. If you’re unsure about how to do this correctly, get in touch with a professional to help ensure your rebuild is a complete success.


If the booster and master problem is not corrected, the lack of stopping condition will continue to worsen, and the brake fluid will be pulled into the engine by the engine’s vacuum system, which will cause damage to your engine.

Have your brake booster checked and installed by a team of auto restoration professionals

When we receive a booster from you for a rebuild and we find brake fluid in the booster, we will always reach out to you and let you know of our findings. This is so that the condition will be corrected prior to the booster being reinstalled after the rebuild.


Power Brake Booster Exchange specializes in rebuilding vacuum brake boosters for classic and sports cars, and with over 30 years of experience, we know exactly how to repair and rebuild brake boosters for all kinds of car models.


Make sure your brake booster is fitted and working the way it should be by booking in for a brake booster service with our team. Book a call with us today or email or call us.


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