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Brake bleeding is the process of adding fluid and removing air bubbles from hydraulic brake lines in automobiles, motorcycles, and even some bicycles. The hydraulic lines cannot function properly if air is present within the fluid or if dirt has somehow worked into the lines, so occasionally brake bleeding is necessary to ensure the fluid can push properly against the pistons that activate the brake pads. This procedure is not particularly difficult, but problems may become evident when performing a brake bleed. Common problems include broken or loose brake lines, a worn piston in the brake caliper, a damaged or worn master cylinder, or any other damaged part in the brake system that allows fluid to escape.
Hydraulic fluid is a silicone-based fluid that is thick enough to absorb a certain amount of air and provide enough pressure in the hydraulic line to activate the pistons of the brake caliper. While mineral oil can be used on smaller brake systems, DOT 4 or DOT 5 fluid is most often used in braking systems. When too much air is allowed into the system — either from an improper bleed or from an opening somewhere along the braking system — brake bleeding is necessary to ensure the braking system can work properly. This usually involves opening a bleed valve or screw at one end of the system and forcing D.O.T. fluid through the opening to the other end of the system.
Some brake systems must be bled from bottom to top: fluid is forced from the lowest point in the system, most often the brake caliper, to the highest point, usually the master cylinder. This pushes air bubbles upward so they can escape the system easily. Other systems are bled top to bottom, which means fluid is added to the top of the brake system and air bubbles are pushed downward. Brake bleeding on automobiles sometimes requires two people, since in many cases the brake pedal must be engaged and disengaged during bleeding, and at the same time the fluid coming out of the brake caliper must be monitored.
When considering doing a brake bleed at home, one should first check the materials and tools requirements of the particular braking system. Some systems are designed specifically for a certain type of brake fluid, and using a different type of fluid can damage the parts of the brake system. Proper tools will also be required for most brake systems. While bleed kits are not expensive or difficult to find, the brake bleed may not be possible without such kits.