Symptoms of a Bad or Failing Vacuum Brake Booster Check Valve

Many vehicles employ a vacuum brake booster in order to provide additional power to the braking system. It's designed to provide consistent flow of hydraulic brake fluid to the brake master cylinder while increasing brake pressure and helping heavy vehicles stop more easily. This component is common on a variety of consumer cars, trucks and SUVs. From time to time, the vacuum brake booster is susceptible to damage or typical wear and tear. This includes the brake booster check valve.

The check valve is designed to suck out air that is trapped in the brake booster without letting additional air enter the cylinder. This protects the brake lines from developing an air bubble which can seriously impact braking performance. This part connects the body of the brake booster to the vacuum hose and is a safety solution that still allows the brakes to work – even if the engine is shut off.

Typically the vacuum brake booster check valve is not inspected during routine maintenance, but there are times when this part can show signs of wearing out or that the booster check valve has failed entirely. Here are a few of those warning signs so you can determine if a potential problem with your vacuum brake booster check valve exists. Remember, these are common warning signs that should be professionally diagnosed by a certified mechanic and properly repaired.

1. Brake pedal is difficult to engage

When the vacuum brake booster check valve is working correctly, applying pressure to the brake pedal is easy and very smooth. When the check valve is not working correctly, the operation of the brakes becomes much more difficult. Specifically, the pedal goes from smooth and soft to aggressive and very difficult to press down. This is due to the excess pressure inside the master cylinder, which the check valve is designed to regulate. The inconsistency of the brake pedal is a warning sign that a potential safety issue exists with the brakes and should immediately be inspected by a certified mechanic.

2. Brakes feel spongy

As the problem with the vacuum brake booster check valve increases, air bubbles will progressively move down the brake lines and to the brakes themselves. In this case, the air that is supposed to be removed by the check valve enters the master cylinder and then into the brake lines. This causes a reduction of pressure inside the brake lines and can cause the brakes to be applied softly. While driving it will feel as if the brake pedal is spongy, however, the brakes will also take longer to stop the vehicle.

This situation calls for an immediate inspection of the brake system. When air enters the brake lines, it typically becomes trapped due to the fact that brakes are hydraulically controlled. In order to remove the air from the brake lines, the brake system will have to be bled. As such, when you experience a problem like this in your vehicle, stop driving it as soon as possible and have the entire brake system professionally examined.

3. Brakes stop working

The worst-case scenario occurs when the vacuum brake booster check valve completely breaks and eventually leads to a brake system failure. Hopefully you never get to this point, but if you do, safely stop the car, have the vehicle towed back home and contact a mechanic who is certified with brake system inspections and replacement. Depending on what actually broke, the repairs can range from simple vacuum brake booster check valve replacement to complete brake system repair and replacement.

The vacuum brake booster check valve is important to your braking system, and acts as a safety feature. It's due to these facts that the problems and symptoms above should not be ignored or put aside for another day. Contact a certified ASE mechanic to inspect, properly diagnose, and make appropriate service adjustments to your brakes.


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Toyota RAV4 L4-2.4L - Brake System Inspection - Sacramento, California
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37 years of experience
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Chevrolet C10 Pickup V8-5.4L - Brake System Inspection - Moreno Valley, California
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Volkswagen Jetta - Brake System Inspection - Moreno Valley, California
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Ford Explorer V6-3.5L - Brake System Inspection - Katy, Texas
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Nissan Frontier V6-3.3L - Brake System Inspection - Flagler Beach, Florida
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