Imagine the amount of brake pressure needed to stop a 4000 lb vehicle.
Probably a lot, right?
Now imagine trying to stop that vehicle all on your own.
Well, that’s exactly how every car works!
All you need to do is press down on your brake pedal – and your car slows down.
But here’s a question —
How can you generate all that stopping power with just your foot?
This is where the brake booster comes into play.
But what is a brake booster?
In this article, we’ll go over everything you need to know about brake boosters.
This Article Contains
(Click on a link below to jump to a specific section)
- What Is A Brake Booster And What Does It Do?
- Types Of Brake Boosters
- How The Common Vacuum Brake Booster Works
- 9 Symptoms Of A Bad Brake Booster
- A Simple Way To Test Your Brake Booster
- The Easiest Solution To Your Brake Booster Issues
What Is A Brake Booster And What Does It Do?
The brake booster is a device used to amplify the force applied on the brake pedal when transferring that force to the brake master cylinder. Brake systems that have them are often called “power brakes.”
The brake booster is used on almost all cars with hydraulic brakes — you won’t see them on vehicles that use pressurized air systems as their primary brake circuits.
Here’s how the brake booster is a vital part of your brake system:
- Your foot applies around 70lbs of force to the brake pedal when you press it.
- That force goes through the brake booster, which amplifies it (often adding 200-300 lb of force) on the master cylinder
- The master cylinder then converts that force to hydraulic pressure
- The hydraulic brake fluid transmits that pressure through the brake lines
- The transmitted hydraulic pressure engages the brake caliper (in a disc brake) or the wheel cylinder (in a drum brake)
- The brake pads (on a brake caliper) or brake shoes (on a wheel cylinder) then use this pressure to generate friction and slow the wheels to a stop
What if modern cars didn’t have a booster?
Without a brake booster, we’d have to press down a lot harder than we usually do to get the brake system to engage. Stopping distances would increase, and emergency braking probably wouldn’t be as effective!
What else are brake boosters known as?
Since brake boosters have been around for a while, they have other names like:
- Power brake booster
- Power brakes system
- Brake vacuum servo
- Brake power booster
- Vacuum booster
The 3 Types Of Brake Boosters
There are three types of brake boosters.
Each brake booster is typically mounted on the firewall in the engine compartment. You’ll find it attached between the brake pedal and the master cylinder:
1. Vacuum Brake Booster
The vacuum brake booster is the most commonly used type.
It utilizes the engine vacuum in naturally-aspirated petrol engines to amplify the pressure applied on the brake pedal.
2. Vacuum Pump
Some road vehicles use a vacuum pump instead of the engine intake manifold.
- Cars with turbo-charged engines
- Vehicles with diesel engines
- Electric vehicles
- Hybrid vehicles
Vacuum pumps can be driven mechanically (from the engine) or via an electric motor (electric brake booster). The vacuum pump is also used in high altitude locations where naturally-aspirated vehicles can’t produce enough vacuum for the brake booster.
3. Hydraulic Brake Boosters
This type of brake booster uses direct hydraulic pressure generated by the power steering pump instead of relying on vacuum pressure.
How The Common Vacuum Brake Booster Works
While there are three types of brake boosters, vacuum boosters are by far the most common. So chances are, the car you’re driving uses one!
Let’s go over how this kind of booster works and how they manage to amplify all that pressure so quickly:
The Mechanics Of A Vacuum Brake Booster
The brake booster has two chambers, separated by a diaphragm.
Some brake boosters come with a single diaphragm, and others have a tandem diaphragm — generally used for larger motor vehicles and trucks.
One of the chambers is linked to the brake pedal, and the other to the master cylinder.
A rod connected to the brake pedal assembly runs through the diaphragm center on its way to the master cylinder piston.
Here’s what happens when your engine is running and you press down on the brake pedal:
- As air rushes through the engine via the air intake valve, each chamber experiences a similar drop in pressure, creating a vacuum (via a vacuum hose).
- Depressing the brake pedal moves the rod forward and opens an air valve.
- Air is drawn into the brake pedal side of the booster, creating atmospheric pressure. A one-way booster check valve (connected to the vacuum hose) ensures that air isn’t drawn into the vacuum intake.
- The pressure difference allows the intake vacuum present in the master cylinder side to pull on the diaphragm.
- As the rod transfers the force applied from the brake pedal, the diaphragm also pulls on the rod, multiplying the force against the master cylinder piston.
This amplification of forces helps you brake the car quickly and easily.
Releasing the brake pedal closes the air valve, stopping the process.
9 Symptoms Of A Bad Brake Booster
You now know how brake boosters work.
But what happens when something goes wrong?
Here are some of the symptoms of a faulty brake booster.
If you spot any of them, take your car to a mechanic immediately:
1. Stiff Brake Pedal Action
A hard brake pedal is often a strong indicator of brake booster failure. 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
If you’ve observed that your car no longer stops as quickly as it used to, you could be having brake booster problems.
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 because of an imbalance in the vacuum chambers of a failing vacuum brake booster.
4. Hissing Noise
Notice a hissing noise when you apply the brakes?
That could be the vacuum booster leaking through the diaphragm or housing, or it could
possibly be a leak in the vacuum hose.
5. Compromised Engine Function
Here’s another potential symptom caused by a vacuum leak.
Let’s say there’s a hole in the diaphragm or a malfunctioning vacuum check valve that draws air into your system.
This air may affect the fuel mix in the engine.
Improper fuel mix affects the correct measure of gas, reducing its cooling action, and increasing the friction and temperature of your engine parts. This 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 valve that will require expensive repairs.
6. Warning Lights Come On
If your vehicle comes with an electronic brake booster, the assembly’s failure can affect the anti-lock braking system (ABS). As a result, your brake warning lights, like the ABS, stability control, or traction control lights, will start to glow.
7. Fluid Leakage
Fluid leaks can indicate a whole bunch of different problems in your car.
If your vehicle uses hydraulic boosters and it’s leaking power steering fluid, there is a strong chance your hydro-boost assembly will soon fail too.
Electronic brake boosters that are housed with the master cylinder can also develop leaks that can compromise their performance.
8. Overheated Hydro-Booster
Sometimes, the fluid temperature in your hydro-booster can shoot up due to environmental conditions. This temperature jump can damage the seals and the spool valve in your hydro-booster – compromising your brake performance.
9. Failing Power Steering
Hydraulic boosters are powered by the power steering pump.
If the power steering fails, the booster will typically fail at the same time.
A Simple Way To Test Your Brake Booster
A lot of these symptoms can be hard to spot since they involve various inner components of your car. But there’s an easier way to check your brake booster — especially if your car uses a regular vacuum booster.
The brake booster design allows it to hold enough vacuum to let you engage the brakes once or twice without the engine running. This helps you stop the vehicle in case of engine failure.
Here’s how you can test your vacuum booster:
- With the engine off, pump the brakes several times.
- Then, press the brake pedal lightly while turning on the ignition.
- The brake pedal should give a bit, then become firm.
- If it becomes stiff or there’s no noticeable change, the brake booster has likely failed.
If you suspect a problem with the brake booster, you should look for a mobile mechanic to help you out. It’s simply far too dangerous to drive to a repair shop without full brake function.
The Easiest Solution To Your Brake Booster Issues
Brake booster problems can show up in different ways, and it’s essential that your mechanic finds out what’s causing the problem.
So, when you’re looking for help with your brake booster issues, make sure that the mechanic:
- Is ASE-certified
- Uses only high-quality replacement parts and tools
- Offers a service warranty
Additionally, as we mentioned, look for a mechanic that can come to you — since you shouldn’t be driving your car to a repair shop with compromised brakes.
And if you’re looking for a mechanic that checks off all those boxes, look no further than RepairSmith to help you.
RepairSmith is a convenient mobile car repair and maintenance solution.
Here’s why RepairSmith should be at the top of your repair solution list:
- Brake booster repairs can be done right in your driveway
- Convenient, easy online booking
- Competitive, upfront pricing
- Expert, ASE-certified technicians will perform repairs
- All repairs and maintenance are performed with high-quality equipment and replacement parts
- RepairSmith provides a 12-month, 12,000-mile warranty for all repairs
And how much would a brake booster replacement cost?
On average, this would cost around $325-$1250, depending on the make and model of your vehicle. For a more accurate estimate, all you need to do is fill this online form.
Your brake booster is one of the most integral components of your car’s braking system.
That’s why it’s always a good idea to make sure your brake booster is in good shape. Stick to a regular maintenance schedule so you can stay safe on the road.
And if you spot any of the symptoms we mentioned, contact Repairsmith to set up an appointment to resolve any brake booster problems!