How to Bleed Your Brakes, and Why It's Important

Alex Palmeri
April 4, 2018

how to bleed brakes

Bleeding your brakes is one of the most important maintenance procedures on your car, and can be a huge help if your brakes feel squishy. Contaminated, old or aerated brake fluid can severely cut into your stopping power.  

Repair shops can take care of a brake bleed, but it’s also something you can do, if you have the right tools, equipment and knowledge.

What is brake fluid?

Brake fluid is a hydraulic fluid that works under high temperature and pressure to move your brake parts. The fluid can’t be compressed and, in most cases, can absorb moisture. 

The force applied by pushing the brake pedal is transferred by the fluid as it travels from the master cylinder, through lines and hoses, to each corner of the vehicle. The fluid then moves the pistons in the brake calipers, which cause the brake pads to clamp down on the rotors and slow the vehicle.  

Brake fluid can also be used with anti-lock braking systems and traction control systems to apply brake pressure using an additional electronically controlled pump.

When do I need to bleed my brakes?

Brake fluid is a maintenance item and needs to be changed periodically. Some manufacturers, like Mercedes, recommend a brake fluid flush every two years or 20,000 miles, while other cars, like certain Honda, Ford and Hyundai models, only call for an inspection and offer no specific interval for replacement. Check your owner’s manual for your car’s recommendations.

Most technicians will suggest a flush every three to four years, or around 50,000 miles, if the owner's manual doesn't list an interval. If you’re having your brake pads or rotors replaced, ask for a brake fluid inspection, as this would be a great time to flush the system as well.  

Bleeding your brakes is also required anytime the braking system is opened. When parts like calipers, wheel cylinders, a master cylinder or a brake hose are replaced, air can enter the system. Since brake fluid by itself isn’t compressible — but air is — any air introduced into the fluid will cause a serious reduction of braking power. When this happens, your brake pedal will feel spongy and may hit the floor with minimal effort.

If a mechanic has worked on your brake system, they should have bled the brakes as well. But if you did some work at home, you’ll want to finish the job.

Here's what to do

The process we’re going to walk through is for hydraulic brake systems. Some newer cars, including many hybrid models, use an electric “brake-by-wire” system that requires a diagnostic tool to complete a brake flush. If you’re not sure which system your car has, check the owner’s manual or call your local dealership.

To perform a hydraulic brake fluid flush, you’ll need the proper brake fluid, a combination or brake bleeder wrench, a clear container, a small rubber hose to fit over the bleeder, some rags, gloves, safety glasses and a second person. (If you don't have a second person handy, there are other ways to make it work.)

Brake fluid is hazardous — it’s a skin irritant, it can cause eye damage, and it can strip the paint off your fender. Be sure to wear gloves and wrap-around safety glasses. Have clean water close to flush any painted surface that the fluid gets on.

The type of brake fluid you will need can be found in your owner’s manual or on the brake fluid reservoir cap. The most common types of brake fluid are DOT 3 and 4, but some newer cars use DOT 5 and 5.1. This chart will show you which fluids are compatible, but remember if your car calls for DOT 5, you can’t use anything else.

brake fluid chart

1. Locate your brake bleeder screws

The first step in flushing the brake fluid is finding the bleeder screws. These are located on the top of most brake calipers. If your car is equipped with drum brakes, the bleeder is located on the wheel cylinder. 

bleeder valve

All car models are different. It’s critical to study the specific procedure for your car before you begin. Manufacturers often add bleeder screws at any point in the system where air bubbles can become trapped. 

These bleeder screws can be found on proportioning valves (which affect the front-to-rear brake bias) or any place the brake lines go from horizontal to vertical and then back down. Don’t let a few additional bleeder screws scare you, just be aware of them and the correct order in which to bleed them.

If you’ve ever had your brake calipers replaced, they may have multiple bleeder screws on them. You only need to open the top bleeders on the caliper you’re bleeding, but you should make sure all the other screws are tightened.

On some cars, it’s possible to gain access to the bleeders without removing the wheels, but in many cases wheel removal is necessary. Proceed with caution and always use jack stands to support your car. If the brake rotors are held on only by the wheel studs, put two nuts back on to hold the rotor in place while you bleed the system. 

2. Get ready to loosen the bleeders

Once you’ve located the bleeders, find the correct size wrench that fits over the bleeder, and always use the box end — not the open end — of the wrench. Bleeder screws are always tight and usually rusty, so you can also buy a six-point brake-bleeder-specific wrench to make the job easier. If you have a penetrating oil like PB Blaster or WD-40, now is a great time to spray the bleeders to ensure they come loose with ease. 

Heat can be used on stubborn bleeders, but if you aren't experienced, this is best left to the professionals. Do not force a bleeder open, as it can break easily and potentially cost you a caliper or wheel cylinder. 

3. Top up your fluid and call over your assistant

Before you open any bleeder screws, make sure your brake fluid is full, and that the venting diaphragm is fully seated in the lid of the master cylinder. Ask your helper to get behind the wheel and make sure the engine is off. You will want to start at the bleeder that is furthest away from the master cylinder — usually the passenger rear.

4. Communicate as you work

Attach your rubber hose to the top of the bleeder and insert the other end of the hose into your clear container, under about a half-inch of brake fluid at the bottom. Before you open the bleeder, have your helper slowly pump the brake pedal two or three times and then push it all the way down and hold. Your helper should signal to you that the pedal is all the way down by saying “Holding down.” 

It’s your turn. Once your helper is holding down the pedal, it’s time to open the bleeder. You should see fluid coming out of the hose, and you may see air bubbles along with the fluid.  

Your helper will feel the pedal go all the way down to the floor, at which point he should say “Floor.” Close the bleeder immediately and then say “Closed.” Your helper can then left off the pedal and say “Up” once it’s in its normal position. 

5. Don’t allow air into the system

If the pedal is released when the bleeder is open, air will get sucked into the system, and you’ll have to start over. That’s why it’s key to close the bleeder quickly.

It’s best to tighten the bleeder to manufacturer’s specifications, but don’t overtighten it. A nice, snug fit usually works just fine.  

6. Top off the brake fluid and repeat

You’ll need to refill the master cylinder after closing each bleeder screw. If you forget to refill it, you run the risk of letting in air and will have to start all over again.

Repeat the bleeding process each wheel, working your way closer to the master cylinder, until the fluid is clean and bubble-free.  

7. Fill the brake fluid at the end

After the bleeding procedure is done, top up the brake fluid to the full line. Make sure you’ve removed the wrench from the last caliper. Clean off any brake fluid with a rag and brake cleaner spray, if you have some. If you removed the wheels, reinstall them and tighten the lug nuts.

8. Take a test drive

It’s always best to take it easy on your first ride after brake work. Drive slow and make sure the brake pedal feels firm and doesn’t easily fall to the floor. After your first drive, double-check the brake fluid level.

If your pedal still feels spongy, you will need to perform a second brake bleed. This isn’t uncommon if you’ve recently opened the system for a repair, so keep your helper nearby.  

What if I don't have a helper?

If you don’t have a helping hand, you can still bleed your brakes in most cases. One method commonly used when flushing your fluid is gravity bleeding. For this, you only need to open your bleeder and let gravity do its job. 

For best results, fill the master cylinder very close to the top and leave the lid off, but do not touch the brake pedal. Open the bleeders one at a time. If no fluid is flowing completely remove the bleeder and make sure the hole through it is clear of debris, then put it back in. It may help to tap on the caliper with a soft mallet. 

Use your clear container and hose, keep track of how much fluid has been flushed at each corner, and refill as you go. This process will likely take longer. Remember to put the lid back on the master cylinder and tighten all bleeders before pumping the pedal.

A second option if you don't have a helper is a vacuum pump. This sucks the fluid from the bleeder screw. You can purchase a hand-operated vacuum pump or a pneumatic vacuum pump that requires a connection to an air compressor. Just attach the supplied hose to the top of the bleeder screw, open the screw and use the pump to remove the fluid. 

These options don't always work when a brake bleed is needed to remove air. In this case, you might need either a helper or a power brake bleeding machine found at most repair shops. If you’re in doubt, let a professional mechanic handle it. Never drive if the brake pedal doesn’t feel right — have the car towed instead. You don’t want to take chances with your brakes.


Alex Palmeri

About the Author

Alex Palmeri worked nine years as a master technician at Mercedes-Benz of Chicago and is currently the foreman at a large fleet garage. He writes about automotive news, maintenance and racing, and runs a YouTube channel called Legit Street Cars.

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