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The Brake Booster Check Valve: What You Need To Know (2021)

March 25, 2021

Ever wonder how a 4000lb vehicle can be brought to a halt just by pressing down on a brake pedal?

You can thank your brake booster for that.

And your brake booster, in turn, is dependent on the brake booster check valve.

But what is this valve, and what does it do? 

In this article, we’ll explore how the brake booster check valve works and how to spot issues with it. We’ll also highlight a simple repair solution if you’re experiencing problems.

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Let’s dive in. 

What Are Brake Boosters?

Most modern vehicles are equipped with a vacuum brake booster to give you a power assist during braking. 

The booster does this by amplifying the forces delivered to the brake master cylinder. The master cylinder pumps brake fluid and converts that force into hydraulic pressure in the brake lines, engaging the brake caliper at each wheel to slow the car.  

The vacuum brake booster operates on a simple pressure differential theory. It comes with several parts that all serve different functions — like the diaphragm, pushrod, dust boots, check valve and grommet, and so on.

For a more detailed look at this, check out our in-depth brake booster guide.

So, where is the booster check valve located?

You’ll find it at the brake booster vacuum inlet, connecting to the vacuum hose from the engine intake manifold.  

What Does The Brake Booster Check Valve Do?

The brake booster check valve is a one-way valve designed to expel air trapped in the vacuum brake booster. 

But why is air in a vacuum booster anyway? 

Air is a primary element in a brake booster. 

The power brake booster typically has two chambers separated by a flexible diaphragm. The vehicle provides a vacuum to these chambers from the engine intake manifold (or from a vacuum pump).

When the driver presses down on the brake pedal, air enters one chamber, creating atmospheric pressure that pushes against the diaphragm. This pressure amplifies the forces from the brake pedal to the master cylinder, stopping your car quickly.

As the pedal is released, vacuum pressure returns to both chambers. 

But where does the booster check valve fit into all this?

Here’s how it works:

1. It Maintains Vacuum In The Booster

The brake booster check valve helps regulate the vacuum in the brake booster. 

When the engine is off or if the vacuum hose has a leak, the one-way check valve ensures no external air enters the vacuum brake booster.

This is a safety feature. 

The power brake booster has to provide the driver enough boost to perform several stops if the engine stops running (remember, no running engine means no engine vacuum via the intake manifold).

That’s why you can still easily depress the brake pedal a few times even when the engine is off, it’s all down to some vacuum pressure being left in the booster.

2. It Releases Pressure Buildup

When the brake pedal is released, all that built-up amplifying pressure in the vacuum brake booster has to go somewhere.

The brake booster check valve helps release that pressure, drawing out all the unnecessary air through the vacuum hose.

What if the vacuum check valve stopped working?

There would be continuous pressure on your braking parts, which could break down from the pressure buildup. 

3. Keeps Air Out Of The Master Cylinder And Brake Lines

Because the check valve sucks out air, it keeps air from entering the brake lines through the master cylinder.

An air bubble in any brake line could have a severe effect on the performance of the braking system.

How Long Does The Brake Booster Check Valve Last?

The brake booster check valve is designed to last as long as the car. 

Because of where it’s located, it rarely gets checked during a brake system inspection or routine maintenance. 

However, that also means that it’s usually harder to notice a potentially failing check valve (unlike thinning brake pads or even a brake fluid leak). 

You’ll only know something’s wrong when the check valve breaks.

To help you stay on top of things, here’s how you can spot a bad brake booster check valve: 

Signs Of A Faulty Brake Booster Check Valve

The symptoms of a bad brake booster check valve are similar to the symptoms of a failed vacuum booster system.

These can include:

1. An Aggressive, Hard Brake Pedal 

This is when the brake pedal feels stiff and challenging to press. 

If the check valve is broken — possibly from a cracked grommet that usually keeps it sealed —  the valve will likely create a vacuum leak.

Reduced vacuum in the booster means less amplifying power. This means you’d need to press down harder on the brake pedal to get the vehicle’s brakes to engage.       

Changes in how your brake pedal feels are usually due to potential issues in the power brakes, and you need to get your vehicle to a mechanic as soon as you can.

2. A Spongy, Squishy Brake Pedal

If the booster check valve is blocked, it won’t suck out air molecules from the vacuum brake booster as efficiently. And if it’s broken, it may let air into the brake booster.

As the bad check valve gradually loses effectiveness, the air that it should regulate can end up in the brake master cylinder, enter the brake fluid, and progress as air bubbles in the brake line.

Any air bubble in the brake fluid is going to reduce the hydraulic pressure that flows back and forth between the master cylinder and brake caliper at each wheel.

When that happens, you’ll find that your brake pedal is soft and squishy and that your car’s braking is less effective. Your brake pedal might even sink to the vehicle floor. 

You’d then have to bring your car to a mechanic to get the power brakes bled of brake fluid to remove these air bubbles. 

3. A Brake Pedal That Has A Swiftly Changing Response 

You might not notice this issue at first, but after a few rapid, successive pumps of the brake pedal, you might realize that the pedal’s gotten stiffer, and your leg is doing most of the work. 

Why does this happen?

If the vacuum check valve is restricted, the power assist will drop off as the vehicle’s brakes are applied in quick succession. 

The block in the valve slows the vacuum return in the brake booster, as the vacuum check valve can’t keep up with the quick brake pumps. 

4. A Vehicle Engine Stall

A vacuum leak from the brake booster can cause unregulated air in your induction system and create a lean fuel mix in the engine. 

You’ll also notice a drop in engine RPM and possible engine stall. 

While this is typically caused by a leaking vacuum brake booster diaphragm, a broken check valve could also be the culprit behind a vacuum leak. 

In this situation, call a mechanic to come to you or have your car towed to a garage. 

But what exactly will your mechanic check for?

Here’s everything you need to know:

What Happens During A Brake Booster Check Valve Inspection?

Here’s how your mechanic can check for a faulty vacuum brake booster check valve: 

Either way, if your vacuum booster check valve is faulty, get it fixed immediately. 

Sometimes the check valve is built into the vacuum hose and can’t be serviced separately. In this case, your mechanic will likely have to replace both the hose and check valve.

Your next question is: how much will it cost to fix a bad check valve?

The Most Convenient Fix For Your Brake Booster Check Valve

The average brake booster check valve replacement will cost around $100-$130. However, you should be aware that repairs might involve more than a check valve fix. 

If your brake booster check valve has been faulty for a while, other brake system parts could have been affected — like possible air bubbles in the brake lines that need to be bled out. 

That’s why you need to find a reliable, trustworthy mechanic to deal with your brake booster check valve issues.  

When you’re looking for a mechanic, make sure that that they:

To keep things simple, look no further than mobile mechanic RepairSmith.   

RepairSmith is a convenient mobile vehicle repair and maintenance solution that checks all those boxes and more.

Here’s why you should opt for them:

For an accurate estimate of your check valve replacement, just fill this online form.