Why Are Brake Bleeders Required?
Brake fluid absorbs water from the atmosphere, lowering its boiling point and causing corrosion. Particulate contamination--microscopic particles of rust and rubber-hastens wear of moving components. This is why manufacturers and mechanics recommend fully flushing hydraulic fluid annually. In addition, whenever the hydraulic system has been opened it must be bled of air. You should also routinely flush and replace your brake fluid every two years. Deposits and debris can build up in the lines over time and decrease the efficiency of your brakes. Regular brake bleeding can also help you spot brake problems that you wouldn’t necessarily notice simply by driving the car.
Brake Bleeding Tools and Their Functionality
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 brake bleeding are also used for purging, where the old fluid is replaced with new fluid, which is necessary maintenance. The process is performed by forcing clean, bubble-free brake fluid through the entire system from the Master Cylinder(s) to the calipers of disc brakes or the Wheel Cylinder of drum brakes). The brake bleeder valve is normally mounted at the highest point on each cylinder or caliper.

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 Master Cylinder          Wheel Cylinder