Brake bleeding is the procedure performed on
hydraulicbrake 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 gasand 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 250ml. Brake fluidis toxic, and must be handled carefully and disposed of properly.
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 cylinders of drum brakes). The brake bleeder valve is normally mounted at the highest point on each cylinder or caliper.
"Please note that the following description of brake bleeding techniques is intended only to provide an overview of the common methods, and is NOT to be used as specific instruction. The actual procedure varies from one vehicle to another and the manufacturers' shop manual procedure should be used."
There are three main methods of bleeding: Pump & Hold (2 variations), Vacuum and Pressure. Pump & Hold generally requires two people, the other methods can be done by a sole person.
*Pump and Hold Method: One person pumps the brake pedal to compress the air, then holds pressure on it. The other person opens the bleeder valve to let out fluid and air, then closes the valve after the pedal has landed (to prevent air being sucked back in through the valve on the upstroke). The process is repeated, sometimes many times, for each wheel. Usually a length of clear tubing is connected to the bleeder valve and run to a container during the process, both to collect the toxic brake fluid and to better view the fluid and bubbles. The master cylinder reservoir must be replenished frequently, for if it goes dry the entire process must be redone. The cover must be left loose so that the fluid may be drawn, but should be in place so that fluid does not squirt out on the return stroke. A block may be placed under the pedal so that it does not bottom out during this procedure, as the master cylinder seals could be damaged by encountering accumulated sediment and / or corrosion.
*Pump and Hold Method, One Person Option: This uses either a "one man" bleeder tool, comprising of a one way valve at the end of a length of tubing which is attached to the bleeder valve, or a special one way bleeder valve, such as the [http://www.russellperformance.com/frequentlyautomotive.htm "Russell Speed Bleeder"] . These methods have the disadvantage that air is sometimes sucked back into the system via the bleeder valve threads, this can be alleviated to a degree by using
teflon tapeon those threads.
*Vacuum Method: The master cylinder is topped off and the cover left loose. A specialized vacuum pump, such as the [http://www.mityvac.com/ "MityVac"] is attached to the bleeder valve, which is opened and fluid extracted with the pump until it runs clear of bubbles. Once again, the master cylinder reservoir level must be maintained. The vacuum method can also draw in air via the bleeder threads, so the bubbles will never clear.
*Pressure Method: A specialized pressure pump, such as the [http://www.motiveproducts.com/ "Motive Product Power Bleeder"] is attached to the master cylinder and filled with fluid. The pump is used to pressurize the system to about 10psi, and the bleeder valves are opened one at a time until the fluid is clear of air. One advantage to this system is that the pump reservoir usually holds enough fluid that running dry is not likely. This is the method most professional shops use, though they use commercial equipment instead of the above mentioned unit.
A new product is the [http://www.brakebleeder.com/about_us.php "Reverse Pressure Method"] by Phoenix Systems. A pump is used to force fluid through the bleeder valve to the master cylinder. This method may have advantages in some cases, however it is not in common usage.
The order in which the wheels are bled is specified in the specific vehicle's shop manual. Typically the wheel furthest from the master cylinder is done first, working towards the wheel closest last.
* [http://www.stoptech.com/whitepapers/howtobleedbrakesright.htm How to bleed brakes.]
* [http://www.lexls.com/tutorials/brake/bleeding.html Another "how to" on bleeding brakes.]
* [http://www.se-r.net/car_info/brake_performance.html A page written by professional racers on maintaining and improving brake performance]
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