The brake booster makes stopping your vehicle easier and safer.
But when it starts to wear out, you’ll feel the difference fast — a stiff brake pedal, longer stopping distance, or even a hissing sound every time you press the brakes.
We’ll explore how a brake booster works and the different types you’ll find in cars. We’ll also cover how to test a brake booster and fix it before it compromises your safety.
This Article Contains:
- What Is a Brake Booster and What Does It Do?
- What Are the Different Types of Brake Boosters?
- 9 Symptoms of a Bad Brake Booster
- Is It Safe to Drive with a Malfunctioning Brake Booster?
- How to Test a Vacuum or Hydraulic Brake Booster
- Can You Replace a Brake Booster Yourself?
- How Much Does a Brake Booster Replacement Cost?
What Is a Brake Booster and What Does It Do?
The brake booster helps amplify the force you apply to the brake pedal. Brake systems with a booster are commonly referred to as power brakes, while the booster itself is often called a power brake booster or vacuum servo. Most cars with hydraulic brakes have a brake booster, but larger vehicles that use air brakes don’t need one.
Among the various types of boosters, the vacuum brake booster remains the most common.
Here’s how a vacuum brake booster works:
- When the engine runs, it creates a vacuum that lowers air pressure inside the booster. This decrease in air pressure on one side of the diaphragm helps transfer force within the booster.
- Pressing the brake pedal allows atmospheric air to enter one side, while the vacuum remains on the other side.
- The pressure difference pushes the diaphragm and rod toward the master cylinder. This adds extra force to your foot pressure, making it easier to stop the car.
- When you release the pedal, the valves close, and the brake booster check valve traps the vacuum inside so the booster is ready for the next stop.
What if modern cars didn’t have a booster?
Without it, you’d have to press much harder on the pedal to slow down, leading to longer stopping distances and less effective emergency braking.
But here’s the thing:
Modern cars use several types of brake boosters to make braking smoother and safer. While they all serve the same purpose, each one works a little differently.
What Are the Different Types of Brake Boosters?
Here’s a quick look at four main types of brake boosters and how they create that extra force:
1. Vacuum Brake Booster
The vacuum brake booster utilizes the natural vacuum created by the engine to amplify the force applied to the brake pedal.
2. Hydraulic Brake Boosters
This one uses power steering fluid instead of engine vacuum. It’s usually found in larger vehicles, such as trucks or SUVs, that require stronger braking power.
3. Electronic Brake Booster
This system utilizes an electric motor to generate brake pressure, enabling it to respond faster and more precisely. It’s ideal for modern cars with safety features such as automatic emergency braking and adaptive cruise control.
4. Electro-Hydraulic Brake Booster
This system combines electric power and hydraulic pressure to assist braking in hybrid and electric vehicles, which don’t produce engine vacuum. It offers strong, consistent braking even when the engine isn’t running.
Regardless of the type of booster your car uses, it can wear out over time. So, it’s essential to know how to tell if a brake booster is bad.
9 Symptoms of a Bad Brake Booster
If you spot any of these brake booster symptoms, take your car to a mechanic immediately:
1. Stiff Brake Pedal Action
Is your brake pedal harder to push than usual?
A failing brake booster loses the ability to amplify the force from your foot, which translates to you having to use more effort when pressing the brake pedal. This lowered force on the master cylinder reduces hydraulic pressure in the brake fluid, making it harder to brake.
2. Increased Braking Distance
When less hydraulic pressure reaches the brake lines and calipers, the brake pads may not clamp the rotors as firmly.
The result?
You need to press the pedal harder to achieve the same stopping power, and your braking distance increases, especially during sudden stops or in emergency situations.
3. High Brake Pedal Position
Here, the brake pedal might travel less than usual (meaning it’s “high”), or it may take extra time to return to its original position after you let go. Either of these conditions can happen due to an imbalance in the vacuum chambers of a failing vacuum brake booster.
NOTE: A high brake pedal position can also result from pushrod and master cylinder issues. So, it’s best to consult a professional for an accurate diagnosis.
4. Hissing Noise
Notice a hissing noise when you apply the brakes?
The noise occurs when air escapes through a damaged diaphragm, cracked housing, or loose vacuum hose connected to the booster.
5. Compromised Engine Function
Here’s another potential bad brake booster symptom caused by a vacuum leak:
Let’s say there’s a hole in the diaphragm or a malfunctioning brake booster check valve that draws air into your system. This air may affect the fuel mix in the engine.
An improper fuel mix (too much air, not enough fuel) reduces the normal cooling effect of the fuel inside the combustion chamber. This raises engine temperature and friction, which can lead to pre-ignition, where the fuel ignites before the spark plug fires.
Pre-ignition can also lead to piston damage or a burnt brake booster check valve that will require expensive repairs.
6. Warning Lights Come On
If your car has an electronic brake booster and it fails, it can affect the anti-lock braking system (ABS). When that happens, warning lights such as ABS, stability control, or traction control will activate.
7. Fluid Leakage
If your vehicle uses hydraulic boosters and is leaking power steering fluid, there is a strong likelihood that your hydro-boost assembly will also fail soon.
Electro-hydraulic brake boosters, located next to the master cylinder, can develop brake fluid leaks that reduce their performance.
8. Overheated Hydro-Booster
Sometimes, the fluid temperature in your hydro-booster can rise due to heavy braking and environmental conditions. This temperature jump can damage the seals and spool valve in your hydro-booster, compromising your brake performance.
9. Failing Power Steering
The power steering pump powers hydraulic boosters. If the power steering system fails, the booster will typically fail at the same time. When that happens, you’ll notice a stiffer brake pedal and heavier steering, especially at low speeds.
If you spot some of these signs but your brakes still function, it’s tempting to delay repairs. But putting off a brake booster repair can have serious consequences.
Is It Safe to Drive with a Malfunctioning Brake Booster?
No, it’s not a good idea.
When the brake booster isn’t working correctly, your brakes lose their power assist — so you’ll need to press the pedal much harder to slow down or stop. That extra effort can make it hard to stop quickly in traffic or during an emergency, which can be dangerous.
It can also put additional strain on other brake components, such as the master cylinder and brake pads. Over time, they’ll wear out faster, which could lead to costly repairs.
Curious to know if your car has brake booster issues?
A few simple tests can help you figure out if it’s actually failing — especially if your car uses a vacuum or hydraulic booster.
How to Test a Vacuum or Hydraulic Brake Booster
Diagnosing a faulty brake booster can be challenging, as it involves several components of the braking system. To avoid a wrong diagnosis, it’s best to have a professional check it.
Still, if your car uses a vacuum or hydraulic brake booster, there are a few simple tests you can try at home to get a better idea of what’s going on.
Testing a vacuum brake booster:
- With the engine off, pump the brakes several times.
- Then, press the brake pedal lightly while turning the ignition on.
- The brake pedal should sink slightly before becoming firm.
If it becomes stiff or there’s no noticeable change, the brake booster has likely failed.
Testing a hydraulic brake booster:
- Turn off the engine and press the brake pedal several times to release any leftover pressure.
- Keep your foot on the brake pedal, then start the engine.
- As the hydraulic pressure builds, the pedal should drop slightly.
If the pedal stays hard, feels uneven, or doesn’t move, there may be a loss of hydraulic pressure, a leak, or an issue with the power steering pump or accumulator.
A failed test means you need professional brake booster repair to get your brakes working properly again.
Can You Replace a Brake Booster Yourself?
No, a brake booster replacement involves working with the master cylinder, vacuum lines, and tight seals around the firewall. If even one part is off, your brakes can stop working correctly — so it’s best to let a professional take care of it.
Once you’ve decided to get it fixed by a professional, the next thing to consider is the brake booster price.
How Much Does a Brake Booster Replacement Cost?
You can expect to pay anywhere from $200 to $1,250 to replace a brake booster, including parts and labor.
Most cars with a vacuum brake booster fall on the lower end of that range, while vehicles with hydraulic or electronic boosters can cost more.
Prices can also vary based on the ease of access to the brake booster and local labor rates. Or if any other components, such as hoses or valves, also need replacement.
Fix Brake Booster Issues Without Leaving Your Home
Regardless of what type of brake booster your vehicle features, it can wear down over time due to leaks, aging components, or regular wear and tear.
While you can spot some brake booster problems yourself, it’s best to let professionals, like AutoNation Mobile Service, ensure your brakes are safe and ready for the road.
With us, you can easily book a repair online, and our certified mechanics will come to your driveway, seven days a week. Even better, every repair includes a 12-month, 12,000-mile warranty.
Book your brake booster service today and get back on the road safely.