How to bleed brakes

How to bleed brakes?

Air in the brake lines is bad news: it means your brakes won’t respond as quickly as they should, and if left untreated it can even cause you to lose control of your vehicle while driving. When the brake pedal starts to feel spongy, or when you can press it all the way to the floor, it’s a sure sign that air has made it into the hydraulic fluid in your braking system. And when that happens, there’s only one solution: you need to bleed the brakes.

If you have a reasonable understanding of how your braking system works, and you feel confident that you can carry out slightly more complex auto repairs, then it’s likely that you’ll be able to drain and change your brake fluid at home. In this step by step guide, we’ll explain how to bleed brakes, so you can save a bit of money and get your vehicle back to peak efficiency in no time.

How do hydraulic brakes work in a car?

Before we explain how to bleed breaks in more detail, it will help to refresh your knowledge of how the brakes in your car work.

A hydraulic foot-brake brake system consists of a master cylinder and four slave cylinders, connected by narrow pipes — or ‘brake lines’ — all filled with a hydraulic liquid. Brake fluid is used in braking systems because it’s not compressible, which means it’s very effective at transmitting pressure. This liquid is usually composed of a mixture of solvents, lubricants, and a few additives to help stop corrosion.

When you press the pedal down, a piston in the master cylinder depresses, forcing the brake fluid along the lines to the slave cylinders at each wheel. The pistons inside these cylinders are forced outwards by the hydraulic pressure, applying the brakes and stopping or slowing the wheels.

How do I know when to bleed brakes? What are the warning signs?

How do I know when to bleed brakes?

Over time, jolts and bumps can mean that air leaks into the braking system. Air in your brake lines is bad news: unlike brake fluid, it’s compressible, which means it effectively lowers the pressure in the system and makes your braking far less effective. That soft, yielding, spongy feeling we mentioned earlier is a sure sign that air has managed to get inside your lines, so if you notice this, you’ll need to bleed the brakes to flush out the air bubbles.

Braking problems can have other causes, too. If your system is particularly damaged by a knock or jolt out on the road, it can cause a leak. When the fluid leaks out, the pressure inside the lines drops and is replaced with air, meaning there may not be enough power to force the brake pads to clamp down on the rotors. If this happens, you need to drain the system, repair the leak, and refill with fresh brake fluid.

Even if the system isn’t damaged, it will still need bleeding every few years or so. While a quality hydraulic brake fluid is designed to last for a long time, it can still become contaminated or corroded over time by oxidisation, moisture, and air. DOT fluids also attract water, which lowers the fluid's boil temperature, and introduces bubbles of air into the brake lines. This in turn reduces the pressure inside the brake system, making the whole system sluggish.

Even if you haven’t noticed a change in your brakes, it’s still sensible to bleed them from the time to time. You should always bleed your brakes if:

  • The car has been sitting parked for a few months.
  • You’ve just replaced the brake pads, or other components of the braking system.
  • You’ve clocked over 25,000 miles, or around every two years — whichever comes first.
  • You’ve been braking abruptly recently, like making an emergency stop.

Bleeding brakes with ABS

You should note that if your car is fitted with an antilock brake system (ABS), then you won’t be able to follow the steps in this guide.

In addition to the conventional bleeding process we’ve outlined here, ABS brakes need to be scanned using a specialist scanning tool and pressure reader, to see if any air bubbles have gotten inside the ABS modulator. If your car has ABS, it’s often easier to seek assistance from a qualified mechanic, as getting air into the actuator can cause serious — and costly — problems.

What you’ll need

To bleed the brakes safely and effectively, you’ll need the following equipment:

  • A bleeder wrench or box end wrench that’s the correct size for your vehicle.
  • A brake bleeder kit, with a clear tube and bottle.
  • Gloves. Brake fluid is corrosive, so you don’t want it to get on your skin.
  • A couple of cans of brake fluid that’s suitable for your car. Check the owner’s manual to see if there’s any specific instructions for this.
  • A jack and axle stands to raise the car up off the ground. You don’t necessarily need to do this, but it will make it easier to access the bleeder nozzles if you do.
  • Old newspapers. You can lay these down on the ground to catch any spills.

Additionally, while it is possible to bleed brakes by yourself, it will be much easier and faster to if you ask a friend to help you. This is because you’ll need someone to press the brake pedal while you open the bleeder nozzle and drain the fluid.

Setting up

Setting Up

The easiest way to access the bleeder nozzles is to remove the tyres. While you can access the bleeder screws by crawling or using a creeper to get underneath the car, it will also help if you can raise your car into the air on jacks. Remember to follow safe lifting practices to prevent you or your friend from getting hurt.

Next, gather your equipment and make sure that you and your friend are fully briefed on what to do. You’ll need to be able to communicate with each other throughout the process to ensure that you don’t accidentally let more air into the brake lines, so make sure it’s quiet enough for you to clearly hear each other.

You also need to work out what order to bleed your brakes in. For a rear-wheel drive car, you should start with the right rear wheel, then the left rear wheel, then the right front wheel, finishing with the front left. Cars with front-wheel drive may need to be bled in a diagonal sequence. Check the owner’s manual for more guidance if you’re unsure.

Bleeding brakes in 11 steps

Step 1: Locate and loosen the bleeder screws

Now that the car is up on the jacks, locate the bleeder screws. These small nozzles are usually positioned just behind the brakes. Take the bleeder wrench and use it to loosen the screws, but don’t remove them just yet.

If the screw is stuck fast, use a lubricant and apply a little bit of pressure to try and loosen it up. Whatever you do, be careful not to force it as this could break the screw, which will require costly professional repairs.

Step 2: Check the brake fluid level

Next, take a look at the brake fluid reservoir. While the location of this can vary depending on the make of car, it’s usually under the bonnet, attached to the master cylinder. If the level is below the ‘maximum’ line, top it up with fresh fluid.

Step 3: Attach the tube

Place the end of the flexible tube from the brake bleeder kit over the nozzle. You want this to be securely over the nozzle, or otherwise the fluid will leak out. Position the container included in the kit at the other end of the tube. It’s sensible to put newspaper down to catch any leaks.

Step 4: Press the brake pedal

Your friend should now get behind the wheel of the car. Once you’re satisfied that the tube is securely in place, shout up to them to pump the pedal three times and then press and hold it to get the fluid moving. It will make it easier and clearer if they say ‘down’ when they’re pressing the pedal and ‘up’ when lifting their foot off the brake. They should hold it down until you give the signal to release.

Step 5: Release the bleeder screw and let out the fluid

While your friend is holding the pedal down, release the bleeder screw by turning it a little more. The brake fluid should squirt out — be careful not to let it spill on you. You should see air bubbles in the brake fluid as it moves through the flexible tube. Your friend should feel the brake pedal go toward the floor as this happens.

Step 6: Close the bleeder screw

Leave the bleeder open for a few seconds, until there are no more bubbles in the tube. Then, while the brake pedal is still pressed down, close the bleeder screw tightly. You can then tell your friend to release the pedal. This needs to be carefully timed: if your friend lifts their foot too soon, air will be sucked back up into the brake line, and you’ll need to repeat the whole process again.

Step 7: Check the brake fluid reservoir

After bleeding the brake and replacing the screw, get up and check the brake fluid reservoir. If the level has dropped, top it up to the ‘maximum’ line. This is an important step: if you don’t top up the reservoir between each bleed, you risk draining all the fluid completely, which would draw in more air through the reservoir and mean you need to bleed the master cylinder too.

Step 8: Repeat the process with each tyre

Repeat the process with every tyre, working in the order specified in the car owner’s manual. Remember to check and re-fill the brake fluid reservoir after bleeding each brake. If the pedal is released before the nozzle is closed, you’ll need to repeat the whole process from the beginning for every tyre, so communicate clearly to avoid confusion.

Step 9: Test the master cylinder for air

Once all four brakes have been bled and you’ve refilled the fluid reservoir for the last time, ask your friend to press the pedal while you look at the master cylinder reservoir. Get them to quickly release and then press down on the brake, while you watch the reservoir carefully.

If you see a lot of movement and air bubbles, it means there’s still air in the brake lines, and you’ll need to repeat the bleeding process for all four tyres. If there’s only a slight movement, the brake system has been properly bled and should now work correctly.

Step 10: Check that the bleeders are closed tightly

Now that you’re done, double check that all of the bleeder caps are closed. Don’t overtighten them, though: this risks breaking them, and you’ll only be making things more difficult for yourself for next time you need to bleed the brakes.

Step 11: Test the brakes

Once the process has been repeated with every tyre, test the brakes out to check they’re now back to full working order: they shouldn’t feel spongy or loose. If they feel as though they’re back to normal, you can take the car out for a quick drive around the neighbourhood to test them out properly — just be sure to drive cautiously until you’re certain that everything is ok.

Hopefully, this guide has left you feeling pretty confident about how to bleed brakes, so you can get back out on the road as soon as possible. If you need to make further repairs to your car, we have a wide selection of braking parts and service accessories available, including discs, drums and shoes, and brake hydraulics. Plus, our search function makes it easy to find what you need: simply search your registration number to shop the parts for your vehicle. You can also find more help and guidance in our auto blog and knowledge hub.