Bleeding a brake is the process of removing old brake fluid from the master cylinder, the hose and the caliper. Bleeding is necessary whenever a brake system connection is loosened, after a component or a hose is replaced with a new one, when a master cylinder or a caliper is overhauled, or when there is a spongy feel to the lever or pedal and it travels all the way to its stop, and where braking force is less than it should be.
Brake bleeding is considered by some to be a bit of a black art – seasoned professionals sometimes have trouble getting a good firm feel in the brake lever, while a first timer may have no trouble at all. One of the problems, particularly with the front brake system, 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 try to rise. Air bubbles can also get trapped, particularly where there are high points in its path, as with the front caliper hose.
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 spanner to fit the brake caliper bleed valve.
Cover painted components to prevent damage in the event that brake fluid is spilled.
So how is it done?