A brake booster is a vehicle system that amplifies the force exerted while you press down on the brake pedal so your vehicle can slow down and come to a stop more easily.
There are two basic things a car needs to be able to do: stop and go. A working acceleration and some good horsepower are all you need to get the car started—but once it’s moving, you want to make sure that the brakes are strong enough to stop the 4,000-pound vehicle. Most cars have sufficient brake systems, often enhanced by a brake booster or power brake.
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What is a brake booster?
A brake booster—otherwise known as power brakes, power brake booster, brake vacuum servo, brake power booster, and vacuum booster—is a part of your car that amplifies the force of your foot on the brake pedal. Through a series of mechanical operations, it literally boosts your brakes.
Brake boosters can be found in nearly every car that uses a hydraulic brake system.
What does a brake booster do and how does it work?
The brake booster of your car is designed to make stopping your heavy vehicle easy. When you push down the brake pedal you exert about 70 pounds of force. Since 70 pounds of force is not enough to bring a 4,000+ pound vehicle to a stop at a safe distance, the brake booster helps you out.
A brake booster raises the force of braking pressure by 200 to 300 pounds, changing the force into hydraulic pressure. This then gets transferred through the brake lines by hydraulic brake fluid, so it can engage the brake pads or brake shoes and bring your vehicle to a stop.
Key Takeaway The brake booster amplifies the force of the brake pedal on the master cylinder, which converts the force to hydraulic pressure and uses that pressure to engage the brake pads or shoes.
MORE: How to check and add brake fluid
Types of brake boosters
Not every brake booster operates in the same way, though. There are three commonly used types of brake boosters depending on the vehicle’s year, size, and model: vacuum brake boosters, vacuum pumps, and hydraulic brake boosters. Here’s an overview of the differences.
Vacuum brake booster
Of all three types, a vacuum brake booster is the most likely type of booster to be in your vehicle. A vacuum brake booster uses an engine vacuum to amplify the pressure on your brake pedal.
Vacuum pump
Vacuum pumps are most likely to be found on vehicles with turbo-charged or diesel engines, electric and hybrid vehicles, and vehicles that can’t produce a sufficient engine vacuum due to high altitudes and lower pressure.
This type of boost can come mechanically from the engine or a motor.
Hydraulic brake booster
Hydraulic brake boosters go hand in hand with your power steering mechanism. Rather than relying on the vacuum pressure of the other two types of boosters, a hydraulic brake booster uses hydraulic pressure from the power steering pump.
How do vacuum brake boosters work?
Since vacuum brake boosters are the most commonly employed brake booster, let’s go over the mechanics of how they work.
Vacuum brake boosters have two chambers and either one or two diaphragms. One of the chambers attaches to the brake pedal and the other one attaches to the master cylinder.
The brake pedal has a rod connected to it that travels through the diaphragm to reach the master cylinder.
When you press your brake, air goes into an air intake valve which then causes a drop in pressure in each chamber—creating a vacuum. The rod attached to the brake pedal moves forward to open an air valve that will draw air into the booster on the side of the brake pedal.
This creates a drop in pressure on one side of the brake booster and an increase in pressure on the other side—this difference in pressure causes the master cylinder’s intake vacuum to pull on the diaphragm. The rod that goes through the diaphragm and is attached to the brake pedal also gets pulled and amplifies the force of the brake pedal.
When you release the pressure on the brake pedal, the air valve closes again and the braking process ends.
Key Takeaway A series of connected elements are essential to creating a vacuum that adds pressure to the braking system, allowing cars to easily stop.
MORE: What to do if you have a faulty vacuum brake booster check valve
How to tell if your brake booster is failing
There are several symptoms to watch for that will signal a failing brake booster. If you do suspect a failed or failing brake booster, don’t hesitate to contact a certified mechanic! Driving without power brakes is dangerous. Here are common symptoms of a problem:
Hard-to-press brake pedal. The booster loses its force-amplifying abilities and requires more force on your brake pedal
Longer braking distance. Your car doesn’t slow down or stop as quickly
Brake pedal position change. Your brake pedal may be “high” if it moves less or takes a longer time to return to its rest position after you release the brakes.
Hissing noises. Hissing sounds when braking could signal a leak in the vacuum booster or vacuum hose.
Stalled-out engine or engine complications. Too much air drawn into the engine will mix with the fuel and air mix already present and can lead to ill-timed engine firing and engine knock that can be very damaging. Too much drawn-in air can also cause the engine to stall out.
Brake warning light is on. The ABS (Anti-lock Brake System), stability control, or traction control warning lights can mean there is a failure in the braking system.
Drastic temperature changes in hydro-booster. If the fluid in the hydro-booster gets too hot, seals and valves in the booster can fail or become damaged. This will affect brake performance.
Power steering failure. Power steering fluid may be leaking which can affect the pump—which powers the hydraulic boosters—or the pump may malfunction.
MORE: 7 common reasons your brakes are grinding
How to test your brake booster
If you suspect that there is something amiss with your brake booster, there is a simple way to check it.
Brake boosters are made with enough space to hold a vacuum for one or two extra brake engagements while the engine is off. To test your brake booster, pump the brakes a few times with the engine off, and then press the pedal down as you turn the engine on.
If there is no change in the feeling of your brake or it immediately becomes stiff and hard to press, it’s a good chance there has been a brake booster failure.
It’s a good idea to call a mobile mechanic if you suspect a faulty brake booster— you don’t want to drive with bad brakes! Be sure you’re hiring a certified mechanic that works with quality tools and parts since “fixing” a brake booster generally means you’ll have to replace it.
MORE: How to decide if you should take your car to a repair shop or a mobile mechanic
How to find affordable car insurance
Depending on the details of your vehicle, brake booster replacements can run between $325 and $1,250. That’s not a cheap repair, but if your current one is failing, you must get it replaced before continuing to drive on it.
If you don’t happen to notice your brake booster is failing until it's too late and you’ve rear-ended someone, you’ll want to make sure your car insurance
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