Automotive Systems/Braking System

Brake bleeding
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.

The brake fluid capacity of a typical automobile is around 500 ml. Brake fluid is toxic, and must be handled carefully and disposed of properly. Most cars use DOT 3 or 4 brake fluids, which may be mixed, but DOT 5 is silicone-based and not compatible with DOT 3 or 4. Note that DOT 5.1 is a higher specification fluid compatible with DOT 3 and DOT 4, but not DOT 5. Most types of brake fluid harm automotive paint and plastics on contact, so special care must be taken when using this fluid: any spills must be immediately cleaned up. Brake fluid is water-soluble so it may be rinsed off with water.

The process is performed by forcing clean, bubble-free brake fluid through the entire system, usually from the master cylinder(s) to the calipers of disc brakes (or the wheel cylinders of drum brakes), but in certain cases in the opposite direction. A brake bleed screw is normally mounted at the highest point on each cylinder or caliper.

''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 five main methods of bleeding: Pump & Hold (2 variations), Vacuum, Pressure, Bench, and Reverse. Pump & Hold generally requires two people, the other methods can be done by a single person.


 * Pump and Hold Method, two people: 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, usually many times, for each wheel. Typically 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. Note that many manufacturers advice against compressing the air first, since it can cause aeration which makes the air more difficult to remove. Instead, they say the bleed valve should be opened first, and then the pedal should be depressed slowly.


 * Pump and Hold Method, One Person Option: This uses either a "one man" bleeder tool, consisting of a one way valve (check valve) at the end of a length of tubing which is attached to the bleeder valve, or a special one way bleeder valve. 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. This method is not recommended because with only one person there is no way to verify if indeed the fluid that is coming out of the bleeder valve has any air present, such as from a bad master cylinder or improper/incomplete bleeding.


 * Vacuum Method: The master cylinder is topped off and the cover left loose. A specialized vacuum pump 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. However, this does not mean there is still air in the brake lines. Using Teflon tape on the bleeder threads will alleviate this issue and allow you to accurately determine when you have evacuated all air from the system.


 * Pressure Method: A specialized pressure pump, 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.


 * Reverse 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.

The order in which the wheels are bled is specified in the specific vehicle's shop manual. Typically the wheel farthest from the master cylinder is done first, working toward the closest wheel last.

If bleeding brakes because of master cylinder replacement the master cylinder is usually "bench bled" before installation. Typically by securing it on the bench, filling it with fluid, connecting fittings and hoses to route fluid from the outlet ports on the master cylinder back to its reservoir, and repeatedly depressing the master cylinder plunger until bubbles are no longer seen coming from the hoses.