The brake booster sits between the brake pedal and the brake master cylinder and multiplies the braking force you apply to the brake pedal. Back in the old days, that’s what was meant when a vehicle was advertised as having “power brakes”, and the brake booster is still often called a “power brake booster”.
The booster is bolted to the firewall in the engine compartment, and the master cylinder is bolted to it. A pushrod extends from the booster into the master cylinder, where it pushes against the piston to apply the brakes. The other end of the pushrod goes through the firewall and clips the end of the brake pedal lever.
Replacement wouldn’t be too difficult except for the location. In the engine compartment, the booster is often tucked under the cowl and hard to get to, and on the other end inside the vehicle, the clips and bolts that attach it are mostly way up under the dash and obstructed by the steering column and various wiring harnesses and pedal frameworks. It would be considered a difficult job for a home mechanic to work on most vehicles.
Types of Brake Booster
Most brake boosters operate with engine vacuum, and their construction is something like a flattened sphere divided into two inner halves by a heavy rubber vacuum diaphragm.
Diesel engines don’t produce vacuum like gas engines, so they either use a dedicated vacuum pump to run the brake booster, or they use a different kind of system. Many use the power steering pump to operate a hydraulic brake booster.
A third kind of brake boost is electrohydraulic, which is a fairly new system design that uses mechanically driven brake fluid under pressure. There isn’t a specific brake booster in this system, but rather an electric pump that pressurizes the fluid, and a system of solenoids and sensors that distribute and monitor the pressurized fluid.
Cost of Brake Booster Replacement
On average, it costs about $420 to replace a brake booster.
For some more specific estimates for booster replacement costs on common vehicles, using $150 an hour as a labor rate:
For a 2006 Honda Accord with a 2.4 liter engine, the labor time to replace the booster is 1.8 hours. A factory booster costs about $185, or a Cardone booster costs about $120. This makes the job about $455 using OE parts, or about $390 using aftermarket parts.
For a 2011 Jeep Grand Cherokee with a 3.6 liter engine, the labor time to replace a brake booster is 1.8 hours. A factory booster costs about $260, or a Cardone part costs about $165. This makes the job about $530 using OE parts, or about $435 using aftermarket parts.
For a 2008 Kia Optima, the labor time to replace the brake booster is 1.3 hours. A factory booster costs about $380, or a Quality-Built part costs about $190. This makes the job about $575 using OE parts, or about $385 using aftermarket parts.
What Causes a Brake Booster to Fail
The main thing that goes wrong with a brake booster is the internal diaphragm leak, which causes a vacuum leak. The diaphragm is made of rubber, which hardens up and loses flexibility with age, and any rubber part that must flex constantly is likely to eventually develop cracks and leak. There are usually a few signs of this. There is an air intake on the nose of the booster where it comes through the firewall under the dash; this often has a filter, and if there is a vacuum leak, air is sucked in there, usually making a hissing noise. This can be either a constant leak or it can be audible only when the brake pedal is pushed. If the leak is bad enough, it can cause problems with engine performance, in the same way that a vacuum leak anywhere in the air intake can cause problems.
Another sign is if the brake pedal feels “high and hard”, and it takes a great deal of force on the brake pedal to stop the vehicle. This is basically what it would feel like if there were no brake booster at all; without the vacuum assist, it takes a great deal of leg strength to apply the brakes. The booster has enough vacuum reserve to apply the brakes two or three times, so one way to get an idea of how it works without vacuum is to apply the brakes two or three times with the engine off; at that point the vacuum should be exhausted, and there will be no power assist to the brakes until the engine is started.
What Else Can Go Wrong
Brake boosters often fail in conjunction with a brake master cylinder failure, if the master fails in a way that leaks brake fluid into the booster. If a brake master cylinder is being replaced for leaks at the rear seal, the booster should be checked closely, and if a booster is being replaced for internal problems, the master cylinder should be inspected for rear seal leaks. Some manufacturers sell the booster and master cylinder together as a package.
Related article: Brake Master Cylinder Replacement Cost
Two other things to rule out before replacing a non-functioning booster are the engine vacuum and the vacuum check valve. If an engine has valve problems, a bad vacuum leak, or general internal wear, it may not generate enough vacuum to operate the booster. This is pretty simple to measure with a vacuum gauge. Also, there is a check valve on the vacuum hose to the booster, usually plugged right into the body of the booster. If this is stuck open, the vacuum in the booster will vary with the exact engine conditions and can cause unpredictable problems. The check valve is also pretty easy to inspect and rule out.
On a hydraulic brake booster, the most common problem is leaks, which can be anywhere in the power steering system. Checking the power steering fluid would be step one. The power steering pump can also fail, but that would be easy to diagnose as it would affect both the steering and the brakes even if full.
Generally not. A minor internal vacuum leak won’t affect brake performance much, but if the booster isn’t working, it can be very difficult to manually stop the vehicle. And a minor leak can become a major leak unpredictably.
Engine vacuum must be good, and the hose to the booster must be sound, as the booster runs on engine vacuum. The booster check valve also needs to be good. These two things are usually checked and ruled out first.
A hissing noise under the dash on the driver’s side when the brakes are applied is a bad sign. That’s the sound of vacuum escaping through a torn booster diaphragm. It’s usually audible for a while before there are any brake performance issues. On a hydraulic system, there would typically be no warning other than a leak if it failed that way.
In this case, losing too much fluid would result in losing both brakes and steering, so no, it is not safe to drive. Depending on how fast the system is losing fluid.