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.
Note: brake fluid is toxic, and must be handled carefully and disposed of properly. The brake fluid capacity of a typical automobile is surprisingly small - only around 250ml - but more than this should be used to ensure a good flush.
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.
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. It may be a good idea to not let the pedal bottom out during this procedure, as the master cylinder seals may be damaged by encountering accumulated sediment and / or corrosion. A good practice is to block the pedal before any work is started. To do this, depress the pedal as far as it will normally go (before you start any work) and place something under the pedal to keep it from going further. This way as you (or your assistant) depress the pedal it cannot go too far and possibly damage the master cylinder.
· 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 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 tape on those threads.
· Vacuum Method: The master cylinder is topped off and the cover left loose. A specialized vacuum pump, such as the 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. Another potential problem with the vacuum method is that the brake pistons will be retracted. This results in the brakes not working until the pedal has been pumped many times which will lower the level of the fluid in the reservoir.
· Pressure Method: A specialized pressure pump, such as the 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.
· Reverse Pressure Method: In this method, 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.
Typically, one starts with the wheel furthest from the master cylinder; almost always the rear passenger wheel, then goes to the rear driver's wheel, front passenger wheel, and finally the driver's front wheel; but see the technical manual for the vehicle in question for proper procedure
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