How to Bleed Motorcycle Brakes

Changing brake fluid (bleeding brakes)

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Bleeding motorcycle brakes is not difficult, but as this task is obviously safety related, it is very important to follow correct procedures. Also, in addition to following correct procedures, the tools used must be of good quality and designed for the task.

Tools Required

  • Dual ended wrench (combined open end and ring type)
  • #2 size Phillips driver bit (depending on the reservoir top)
  • Bleed bottle with rubber connection hose
  • Ratchet driver with 1/4 inch drive socket
  • Small hammer (1/2 pound)
  • Supply of water (to clean any spillage)
  • Brake fluid—check the shop manual for the correct grade, or with the previous owner if nonstandard fluid was used. Do not mix different grades.

Brake bleeding is part of the regular maintenance program. However, if the system has been completely emptied as when replacing hoses, the procedure is slightly different.

To cover both complete fluid replacement and periodic maintenance, we will assume the system is empty.

Protecting the Paintwork

The first part of the process is to remove the reservoir top or cap and fill the reservoir. Fill to slightly below the top edge. However, it is good practice to place a number of absorbent rags around the reservoir to soak up any spillage. It is particularly important to protect any paintwork (gas tank, front fender, and frame) from spillage.

Underneath the reservoir top should be a rubber sealing diaphragm. This diaphragm keeps the brake fluid separate from the atmosphere (air is allowed into the top of the reservoir to replace lowering brake fluid levels).

Brake fluid must also be poured into the bleed bottle, ensuring the outlet pipe is below the surface of the fluid. The wrench (ring end) should be placed onto the nipple first, followed by the bleed bottle's rubber hose.

The Brake Bleeding Process

With the various items in place and the reservoir full, the process of bleeding can begin. With an empty system, the bleed nipple should be opened about 1/3 of a turn and the lever repeatedly pumped to send fluid into the brake hose. It is very important to keep topping-off the fluid reservoir during this stage of the process as air can enter the system.

As the brake lever is being actuated, a series of air bubbles will be seen coming from the end of the bleed bottle's tube, below the fluid level. These bubbles are the air within the system being forced out by the new brake fluid.

When the mechanic is satisfied that the brake system is full of fluid, the final bleeding phase can be done. This is the phase that would normally be done during a service, rather than when the system is empty.

No Air Bubbles

The brake lever should be pumped approximately three times and then held in (brake applied position). The bleed nipple should now be closed and the pumping repeated. Next, with the lever in, the bleed nipple should be opened then closed. This process should be repeated until no air bubbles can be seen leaving the end of the bleed tube (the fluid level must be topped off periodically). That is: pump and hold the lever in, open the bleed nipple, then tighten the nipple and release the lever.

Note: Some brake calipers have more than one bleed nipple. When bleeding this type of system, the nipple furthest away from the reservoir should be bled first.

At the completion of the brake bleeding process, the lever should feel firm when pressure is applied with no sponginess (nipple closed).

Great care must be exercised when removing the bleed bottle as the rubber hose will contain brake fluid. These hoses (being made from rubber) tend to spring out and send a small amount of brake fluid into the air. This fluid will seriously damage paint (wash off with lots of water) and cause serious damage to the mechanic's eyes—read the safety instructions before using.

After the bleed bottle has been removed, a proprietary brake cleaner can be sprayed onto the caliper and the bleed nipple, to remove any spilled brake fluid. The rotor must also be wiped clean with brake cleaner to remove any fluid or fingerprints, and the dust cap replaced onto the nipple.

The brake fluid reservoir should be topped off one final time and the top replaced and secured.