The brake booster is the heart of the power brake system. When you step on the brake pedal, the booster multiplies the force from your foot, thereby reducing the effort needed to stop the vehicle.
These days, we take power-assisted brakes for granted. This technology wasn’t always commonplace—until power brakes became the norm in the late 1960s, many drivers relied solely on their leg muscles to provide braking assist.
With that said, when the brake booster fails, you’re likely to notice it right away, as this issue usually produces one or more noticeable symptoms. Obviously, because the booster affects your car’s braking ability, you’ll want to address any problems right away.
If your car has a bad brake booster, you’ll likely notice one or more of the following signs:
A hard brake pedal is the most common sign of a bad brake booster. When the booster fails, you no longer have braking assist, which means you’ll need to exert more effort to apply the brakes.
Without the booster providing braking assist, you’ll have to work harder to apply the brakes. This typically results in an increased stopping distance.
A vacuum-operated booster that’s leaking through the housing or diaphragm can create a hissing noise. You may only hear the noise when applying the brakes.
Hydro-boost brake boosters use power steering fluid to provide braking assist. As such, a leaking hydro-boost unit can develop visible fluid leaks. Electronic brake booster assemblies that are housed with the master cylinder can also develop leaks.
An issue with the electronic brake booster assembly can affect anti-lock braking system (ABS) operation. As a result, a failure of the assembly has the potential to trigger warnings, such as the ABS, traction control, and stability control lights.
It’s also possible for a leaking, vacuum-operated brake booster to cause the engine to run lean, which can trigger the check engine light.
As was mentioned, the brake booster reduces the effort the driver needs to apply the brakes. The booster does this by applying force to the master cylinder—a device that distributes pressurized fluid to activate the brakes.
There are three primary types of brake boosters:
Most brake boosters are vacuum-operated. With this design, an internal diaphragm separates the two sides of the booster. The two sides are sometimes called the vacuum chamber and the working chamber.
When the brakes are released, there is an equal amount of vacuum in both chambers. Applying the brake pedal operates a control valve, which allows atmospheric pressure to enter the working chamber. As a result, a pressure difference is created that causes the booster’s pushrod to apply force to the master cylinder, creating brake assist.
Some vehicles use a hydro-boost brake booster. With this type of booster, the power steering pump creates hydraulic pressure to provide braking assist. Pressure from the pump acts on a power piston inside the hydro-boost assembly. The piston then pushes an output rod that applies force to the master cylinder.
Electronic brake booster assemblies are becoming increasingly common. You’ll often find this design in hybrid and electric vehicles that have zero (or limited) engine vacuum and electric power steering.
There are a variety of electronic brake booster designs in use today. Each uses a collection of sensors (i.e., pedal stroke sensor, speed sensors, etc.) to determine vehicle operating conditions. An electronic control unit (ECU) uses that information to activate the electronic brake booster.
The booster and master cylinder then create the hydraulic pressure needed to provide braking assist.
Often, the electronic brake booster is integrated into an assembly with the master cylinder and ECU. Other components may be part of the assembly as well.
The steps for testing a brake booster will vary depending on the vehicle and the booster’s design. The information below is generic and for entertainment and educational purposes only.
Be sure to follow the factory repair information for your specific application.
With that in mind, let’s discuss general brake booster test methods for a traditional, vacuum-operated unit.
Vacuum-operated brake boosters have a reserve to provide braking assist if engine vacuum is lost. You can check that the booster is operating properly and has an adequate vacuum reserve (indicating it’s free of leaks).
Here’s how it’s done:
Tip: Because the booster relies on engine vacuum, an inadequate vacuum supply can mimic a bad booster.
Otherwise, the booster is likely faulty.
The tools needed to replace a brake booster will vary, depending on what type of car you have.
In general, however, you’ll need:
Before we begin, keep in mind: all vehicles are different. The information below is generic and for entertainment and educational purposes only. Be sure to follow the factory repair information for your specific application.
Repair manuals, such as those from Chilton, are useful, but a subscription to a repair database is even better. ALLDATA and Mitchell 1 both have single-vehicle subscriptions for DIYers that provide detailed factory repair information.
You can learn more about accessing quality repair information in this article.
Note: Check the brake pedal height and free play after replacing the master cylinder. If necessary, consult a repair manual or repair database for the proper adjustment procedure.
It’s not a good idea to drive with faulty brake boosters. Thankfully, it shouldn’t be too difficult to find a replacement here at CarParts.com.
You can get brand-new brake boosters without taking one step outside your door. To get started, visit our website and use our vehicle selector to narrow down our catalog to your model year’s compatible parts.
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Don’t let a bad brake booster keep your vehicle stuck in your garage for too long. Order your new brake components today!
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.
View Comments
Hi Mia, I have a fun repair issue with my 2004 Hyundai Sonata GLS. It operated like its in full boost only. Touch the pedal while running and it hammers on the brakes. I disconnected vac to the booster to verify brake operate hard with no boost; did the pedal test at shut off; ran about 3 pumps, and pedal goes down when started. I'm thinking it could be the brake booster maybe a stuck internal valve, or what else could make this a super-booster? pedal not adjusted right? it seems to engage at about the normal depression. any thoughts where to look next?
Hi Reno,
Just to clarify, when you removed the vacuum source from the booster, did the brakes release? The pedal will undoubtedly be hard, but from what you describe, the brakes sound like they're locking up/being grabby, and I'm wondering if that stopped with the vacuum hose removed.
I have a 1999 4runner that the brakes go almost all the way to the floor and hardly not enough pressure to stop the car. It's not leaking fluid, any ideas?