How To Bleed
Brake Discs

Good maintenance is vital for prolonging the life of your bike. Knowing how to take care of the brake system is just as important as looking after the tyres, bodywork and pedals. Having a top-quality braking system means you have the stopping power you need to have total control over your ride. But with great power comes great responsibility, so it's really important to know the proper bleeding procedure for keeping your brakes in tip-top condition.

Our team knows a thing or two about brake system maintenance and TLC, so we put together this expert guide to brake bleeding, so you can make your bike feel as good as it looks.

What are brake discs?

First things first, let's get to know the brake system on your bike. We're specifically talking about disc brakes here - some bikes also have 'rim brakes'. The names refer to where the actual stopping or braking force is applied. While rim brakes apply force to the bike wheel's rim, disc brakes work with a disc (or rotor) that's attached to the wheel hub.

Brake pads grip the brake discs, using a brake calliper. Disc brakes are known for their great stopping power, and offer a good amount of control in all weather.

brake pads

What does 'brake bleeding' mean?

The process of bleeding a brake refers to the method of removing air bubbles from your bike's hydraulic system brake fluid. This is actually the same process you'd follow to replace brake fluid when it's time for a change. So, if you need or can do both at once, you may as well save yourself some time!

Front brakes

How do you know when to bleed disc brakes?

The typical advice is to bleed your bike brakes yearly or every two years at least, as a rule of thumb. Manufacturers of TRP or Shimano hydraulic disc brakes also recommend that riders who cycle on uneven or demanding terrain bleed brakes more often. This is also the case if you cycle particularly quickly or aggressively.

One of the telltale signs that you might need to start the brake bleeding process is if you notice any change in performance or loss of power. One way to check this is by putting one finger around your handlebar grip, and using the other hand to gently pull the brake lever back. If the brake lever touches your finger, it's probably time to check and bleed the brakes. Of course, this isn't a foolproof test, and if you've had your brake lever adjusted (or it was particularly close to the handlebar in the first place), this won't be the most accurate method for checking.

If you notice that the brake calliper pistons feel steady and are working correctly, but the actual brake feels soft, it's probably time for a bleed.

What are the benefits of brake bleeding?

Remove the air from your brakes

Hydraulic braking systems use incompressible fluid to operate. When you press the brake lever, brake fluid then sets in motion the brake caliper pistons, which then move the brake pads. So essentially, this incompressible brake fluid is the key to all the parts of your brake system working together smoothly. If there's air bubbles in there, this disrupts the whole process.

If there are too many trapped air bubbles in your brake fluid, you might experience what is sometimes referred to as a 'spongy brake'. This is when the brake lever has to be pulled hard (or all the way to the handlebar as described above) in order to engage. With this much movement you might find that your stopping power is hugely reduced. To have complete faith in your brakes and for them to be performing at their best, you want the brake lever to feel firm to the touch, which means getting rid of any air inside.

Replace or change your brake fluid

Just like with car maintenance, every now and then your bike's brake fluid reservoir needs a bit of a change. As time goes on, and as your bike racks up the miles, the brake fluid inside gets older. It can get contaminated from your rides too, with air bubbles as well as dirt, mud and water.

Replacing the old brake fluid not only helps you get more control over braking power, but can prolong the life of your brake pads, brake fluid reservoir... basically your whole brake system! If you're not sure which brake fluid to use (the two main types are Mineral Oil and DOT brake fluid), refer to your bike's owner manual or visit local bike shops for advice.

Shimano brakes use mineral oil brake fluid, rather than DOT brake fluid. While mineral oil is more gentle on the skin in case of spillage (and is generally less likely to suck air from the atmosphere), it's also less durable at high temperatures. Make sure you check you're using the correct fluid before you start bleeding.

How to bleed disc brakes

In a nutshell, to bleed brakes correctly you need to use the caliper and syringes to remove air bubbles from the fluid chamber and replace old brake fluid to make sure your braking system is clean, and the brake lever feels firm to the touch.

While it's always an option to visit your local bike shop when it's time to bleed your brakes, it's also possible to sort it all at home. Arm yourself with the right kit, your owner's manual, a bit of patience and about half an hour, and you can get your brakes back into prime condition.

What you'll need to bleed bike disc brakes

  • Bleeding Kit - including bleed adapter (with o ring), rotor tool, bleed cup, bleed adaptor, syringes

  • Bike Tools (i.e. wrench, hex or Allen keys etc) - this should include a screwdriver for removing bleed port screws

  • Protective gear - Including gloves to make sure you don't get brake fluid on your skin

  • Cleaning cloths or paper towels

  • Bleed block - to hold the brake caliper pistons in place

bike bleed tool

Step by step guide to the bleeding process for bike disc brakes

This is the bleeding process for hydraulic braking systems such as Shimano disc brakes, and includes a step for replacing old fluid with mineral oil brake fluid (rather than DOT fluid). If your brakes start feeling spongy, it might mean the fluid level needs topping up, or you might need to dispel excess air bubbles. Here's how to bleed hydraulic disc brakes:

  1. Make sure your bike is in a stable position - preferably mounted on a workstation or in a bike stand. Although brake bleeding is a simple process, you do need to make sure you've got enough space to manoeuvre and a clean surface for removing and cleaning parts.

  2. Using your bike tools, remove the wheel to access the brake pads. Use a piston press (or a flat screwdriver) to push the pistons back, remove the brake pad retainer clip and pull the brake pads out. Now it's time to put in a bleed block.

  3. Use an Allen key to loosen the brake lever clamp, twist it to horizontal and then tighten back up. Use a smaller Allen key to take off the bleed port screw (at the top of the brake lever reservoir), and remove the o ring. If you can't see the o ring to remove, it may come out with the cap.

  4. Take your bleed cup and connect it to the bleed port, taking care not to tighten too much and damaging the threads.

  5. Remove the cover from the bleed nipple (on the caliper). Hook up a bleed hose to a syringe filled with mineral oil (take care to make sure there's no trapped air bubbles in there) and attach the other end to the bleed nipple. If your bike has non-series Shimano brakes (or others) you may need to use an Allen key on the bleed valve instead).

  6. Push the syringe to send the oil through the brake system, making sure to stop before any air enters into the caliper. Then close up the valve.

  7. Take off the syringe, and point the hose into a bag or bucket. For extra security when using a bag for brake bleeding, you can use a tie or string to secure the opening, but make sure you're not cutting off the hose.

  8. Open the valve/nipple and drain about half the oil in the bleed cup. Close the valve, and squeeze the brake lever 2-3 times. Then re-open the valve, squeeze the lever and close the valve before letting go of the lever. You should repeat this a few times to release any air.

  9. Close up the bleed nipple and take off the hose. Tap the brake hose to get rid of excess air inside, and give the brake lever a few good squeezes.

  10. Put the plug back in the bleed cup, take it off and put the bleed port screw (as well as the o ring) back in place. Take off the bleed block, and make sure to give the caliper a good clean with a wipe or paper towel. Install brake pads back into their position, and put the wheel securely back on. Test the brake lever to double-check that it's firm and feeling smooth.

For more advice on bike maintenance or cycling inspiration, head to our Bike Knowledge Hub.

brake cleaner

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