Bleeding Brakes - Everything You Need To Know On How To Bleed Brakes

How to bleed brakes?

Bleeding brakes is a sure-fire way of improving the response and safety of your car's braking system. It's an essential part of car maintenance that can be efficiently completed with a little bit of car knowledge. Our handy guide takes you through a step-by-step process on how the hydraulic braking system works and how to bleed your car's brakes. 

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How Do You Bleed Brakes?

Air in your car 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 feel confident that you can carry out slightly more complex auto repairs, you'll likely be able to drain and change your brake fluid at home. In this step-by-step guide, we'll explain how to bleed brakes to save money and quickly get your vehicle back to peak efficiency.

  • How Brakes Work in a Car - The Hydraulic Car Braking System
  • How Do I Know When to Bleed My Car's Brakes?
  • Bleeding Brakes With ABS
  • What You'll Need to Bleed Your Brakes
  • Preparing Your Vehicle for Brake Bleeding
  • How to Bleed Your Car's Brakes - 12 Easy Steps
  • Brake Bleeding Know-How

How Brakes Work in a Car - The Hydraulic Car Braking System

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 car 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 is 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 car brake 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 My Car's Brakes?

Over time, jolts and bumps can mean 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.

Other Braking Problems

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 must 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 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, lowering the fluid's boil temperature and introducing air bubbles 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 occasionally. You should always bleed your brakes if:

  • The car has been parked for a few months.
  • You've just replaced the brake pads or other braking system components.
  • 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), you won't be able to follow the steps in this brake bleeding guide.

In addition to the conventional bleeding process outlined here, ABS brakes must 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 Your Brakes

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.
  • A pair of 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 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, it is possible to bleed brakes by yourself, but it will be much easier and faster if you ask a friend or family member 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.

Preparing Your Vehicle for Brake Bleeding

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 ensure 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 hear each other clearly.

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 need more clarification.

How to Bleed Your Car's Brakes - 12 Easy Steps

Step 1 - Locate and Loosen the Bleeder Screws

Now that the car is up on the jacks, you need to locate the bleeder screws. These small nozzles are usually positioned just behind the brakes. Use the bleeder wrench to loosen the screws, but don't remove them just yet.

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

Step 2 - Check the Brake Fluid Level

Next, take a look at the brake fluid reservoir. While this location can vary depending on the make of the 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 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 to them to pump the pedal three times and press and hold it to get the fluid moving. It will be more straightforward if they say 'down' when 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 moving 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, 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 refill 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, 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 bleeder caps are closed. Don't overtighten them, though: this risks breaking them, and you'll only be making things more difficult for yourself the 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 to check they're back to complete working order: they shouldn't feel spongy or loose. 

Step 12 - Take Your Car For a Test Drive

If they feel like 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 confident everything is ok.

Brake Bleeding Know-How

Hopefully, this guide has left you confident about how to bleed brakes so you can get back 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 and pads, 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 for the parts for your vehicle. You can also find more help and guidance in our auto blog and knowledge hub.

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