Procedures for Bleeding Brakesby Chris Stevenson
Auto brake systems contain brake fluid lines that lead from the master cylinder to each wheel. The brake fluid flows to a caliper or wheel cylinder and shoves the pads or brake shoes against the discs or brake drums, causing friction to slow or stop the vehicle. Since brake fluid does not readily compress, it is well suited to translate "foot power" when applying the brakes. After a complete brake job or the discovery of trapped air, several procedures can be used to "bleed" brake lines and components.
Manual Brake Bleeding
Manual brake bleeding requires an assistant to help perform the task. In this procedure, the master brake cylinder is filled and capped. The repairperson starts at the wheel farthest away from the master cylinder. While the assistant pumps up the brake pedal (two or three times) and holds a steady pressure on it, the repairperson loosens the brake cylinder (or caliper) bleeder valve, using a brake wrench, then closes it off quickly when the assistant's foot has depressed the brake pedal to the floor. This allow a steady flow of hydraulic brake fluid to pass and escape, removing the air bubbles. The procedure is performed on each wheel in the same fashion and repeated as necessary until each brake line is cleared of air. The master cylinder must be replenished during and after each bleed.
Power bleeding requires only a single repairperson; a pressurized tank replaces the assistant. The power-bleed device has a universal master cylinder cap (lid) fitted over the master cylinder reservoir that seals the system air-tight. Pressurized brake fluid from the tank (operated by a valve) enters the master cylinder and flows through the lines. The repairperson only needs to open a bleeder valve and re-tighten it after all the air bubbles have escaped. The power-bleed tank unit can be found in many automotive repair shops due to its reputation for fast and error-free bleeding.
Master Cylinder Bench Bleed
The master cylinder can have trapped air in its body, requiring it to be bled like a wheel cylinder. In this procedure, the master cylinder can be off the vehicle and held in a vise, or remain on it to perform a manual bleed. By pushing a screwdriver repeatedly into the front master cylinder piston, then holding it firm, the repairperson uses a brake line wrench to temporarily open (crack) the brake line fitting, expelling brake fluid. The procedure must be performed on each of the master cylinder brake line fittings until no air escapes from the lines. Note: The power-bleed unit can also be used in this application. There will be no need to pump the master cylinder piston, since pressure already exists. Once under pressure, bleeding the system requires only momentarily loosening the line fittings, then tightening them again.
- photo_camera disc brakes image by KtD from Fotolia.com