Hydraulic Disc Brakes

Cycle Guides > Hydraulic Disc Brakes

This Guide is intended to help you understand the function of hydraulic disc brakes and explain how to replace brake fluid. These brakes operate in much the same way as a cars disc brakes work. They utilise a calliper that is usually attached to the forks (near the drop out). They slow or stop the bike when you apply pressure to the brake handles - this pressure is transferred to callipers via fluid which forces brake pads onto the discs (rotors). The friction causes the bike to slow/stop. More friction is gained by the use of soft compound pads which will give you more stopping power but won't last as long. Harder compound pads tend to last longer and are more suited to wet and muddy conditions.

Bleeding and Replacing Brake Fluid

It's a good idea to replace the brake fluid now and then as the fluid becomes contaminated with moisture and dirt over time and with heavy use. It's important to use the manufacturer recommended brake fluid as some use different grades and some have a completely different content (some use mineral oil, others use automotive fluid). You should take care not to contaminate brake pads with the fluid and try not to get it on the callipers or brake levers. Before you start to replace the fluid, the bike needs to be upright and stable (preferably on a bike stand). Don't try to change the fluid upside down as this will result in air getting into the cables. Make sure there are no leaks from cables or the calliper before you start (if there are you may need to replace some parts).

This guide is meant as an outline for order in which to carry out the replacement of brake fluid. You will need to follow the brake manufacturer's instructions to know the exact process as they are model and brand specific.

You're probably going to need:

  • A Torx driver (Usually T10)
  • A 4mm Allen Key
  • A 7 or 8mm Spanner/Wrench
  • Some Brake Fluid (for type see manufacturer's instructions)
  • A Small Flat-Headed Screwdriver
  • A Bleed Pipe
  • A Plastic Bag
  • An Elastic Band
  • Some Old Brake Pads/Cardboard/Wood Spacers
  • Some Tissue
  • Rubber/Latex Gloves (needed for use with toxic automotive brake fluid)

Hydraulic systems have reservoirs which can usually be accessed by removing a cap from the top of the brake lever. This reservoir and bladder is an important feature of the system because when the brake fluid heats up it expands and needs somewhere to go. The reservoir acts as a release for the excess fluid to prevent it from splitting a cable.

Bleeding: This is a process to allow the trapped air to escape from brake cables and callipers. If you have air in the system it will result in a much slower braking response as air can be easily compressed whereas brake fluid can't. Take care to keep all parts of the system clean, especially the reservoir and the pads. Take off the wheels being careful not to damage the discs as they pass through the calliper. Remove the brake pads and replace them with either cardboard/wood spacer or some old pads. The bike should be positioned so that the cable is continually facing uphill between the calliper and the reservoir. The top surface of the reservoir should be parallel to a flat ground (if not adjust the brake levers). Take the bleed pipe and attach it to the bleed nipple on the calliper. Use the elastic band to attach the plastic bag to the other end of the pipe. Make sure you wipe any dirt away from the brake lever and around the reservoir. Unscrew the reservoir tank cap and remove the cover and bladder. Fill the reservoir tank to the top with the correct brake fluid. Unscrew the bleed nipple with one turn. Repeatedly pump the brake lever. The fluid in the tank will bubble and the level will probably drop. Make sure the reservoir is topped up as the level drops. Tap the cable to dislodge any trapped air. Fluid will eventually start to come out of the bleed nipple - close the nipple. When you test the brakes they should eventually become firm when applied. If not, open the bleed nipple and continue. When firm, hold the brake lever and keep the pressure applied. Use the wrench to loosen the bleed nipple, opening and closing in the space of a second whilst looking at the fluid passing through the bleed pipe. This should expel any remaining air bubbles. Let go of the brake lever and top up the reservoir. Pump the lever once again until firm. Top up the reservoir. The system should now be free of air. Replace the bladder and reservoir cap. Be ready to wipe any overflowing fluid from the lever - this is normal. Screw the cap on securely. Take off the bleed pipe from the nipple and wipe any fluid off the calliper. Replace the pads and wheels. Finally check the brakes work.

Replacing Brake Fluid: Carry out all the steps mentioned above but leave the nipple open when pumping the levers, and keep topping up the reservoir. Eventually the new fluid will replace the older fluid.

That's it, your brakes should now be air free and fully topped up with new fluid.