BRAKE BLEEDING Brake bleeding is the procedure performed on hydraulic brake systems whereby the brake lines (the pipes and hoses containing the brake fluid) are purged of any air bubbles. This is necessary because, while the brake fluid is an incompressible liquid, air bubbles are compressible gas and their presence in the brake system greatly reduces the hydraulic pressure that can be developed within the system. The same methods used for bleeding are also used for purging, where the old fluid is replaced with new fluid, which is necessary maintenance. The brake fluid capacity of a typical automobile is around 500 ml. Brake fluid is toxic, and must be handled carefully and disposed of properly. Most cars use DOT 3 or 4 brake fluids, which may be mixed, but DOT 5 is silicone-based and not compatible with DOT 3 or 4. Note that DOT 5.1 is a higher specification fluid compatible with DOT 3 and DOT 4, but not DOT 5. Most types of brake fluid harm automotive paint and plastics...
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