Bleeding a brake is the process of removing aerated brake fluid from the motorcycle's master cylinder, the hoses/pipes, modulator (ABS models) and the brake caliper(s).
Bleeding is necessary whenever a motorbike's brake system hydraulic connection is loosened, after a component or hose is replaced with a new one, when the fluid is changed, or when there is a spongy feel to the lever or pedal, and where braking force is less than it should be, and it is not due to any mechanical fault in the system (i.e. a sticking piston in the caliper, or a pad that is not moving as it should due to corrosion, for example on the pad pin).
Leaks in the system may also allow air to enter, but leaking brake fluid will reveal their presence and warn you of the need for repair.
How difficult is it to bleed my bike’s brakes?
Brake bleeding is considered a bit of a black art – technicians sometimes have trouble getting a good firm feel in the brake lever or pedal, while a first-timer may have no trouble at all.
One of the problems, particularly with the front brakes, is that you are working against natural principles – science dictates that air bubbles in a liquid will rise to the top, but the process entails pumping the brake fluid and any air bubbles it contains down, from the master cylinder at the top to the bleed valve in the caliper at the bottom, so while the fluid is moving down the air bubbles will want to rise. Air bubbles can also get trapped, particularly where there are high points in its path, and when there are extra components and pipes as in the ABS system.
What equipment do I need to bleed my brakes?
To bleed the brakes using the conventional method, you will need some new DOT 4 brake fluid, a length of clear flexible hose, a small container partially filled with clean brake fluid, some rags, and a ring spanner to fit the brake caliper bleed valve.
Bleeding kits that include the hose, a one-way valve and a container are available relatively cheaply from a good auto store, and simplify the task. You also need a block of wood as a support for the fluid container.