BLEEDING BRAKES



BLEEDING YOUR BRAKES

Bleeding your brakes is performed on hydraulic brake systems to clear the brake lines of any air bubbles, if you have air bubbles present in your brake lines it reduces the hydraulic pressure making your braking system much less effective.

The brake fluid capacity of a typical car is around 250ml, you need to handle brake fluid carefully as it is toxic, it should also be disposed of properly.

Most cars use DOT 3 or 4 Brake fluid, these can be mixed, whereas DOT 5 is silicone based and not compatible with DOT 3 or 4, a word of caution, most types of brake fluid can harm paint and plastics on contact, so be careful, brake fluid is water soluble and can be rinsed with water.

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

There are four main methods of bleeding, Pump & Hold (2 variations), Vacuum Pressure, and Reversed Pressure, pump & hold generally requires two people, the other methods can be done by one person.

The Pump and Hold Method with two people, one pumps the brake pedal to compress the air, then holds the pressure on the pedal, the other person opens the bleeder valve to let out the fluid and air, the valve is then closed after the pedal has hit the floor, this process is repeated until the system is clear of air bubbles and clear brake fluid flows freely, this process is carried out for each wheel, the master cylinder should be checked frequently to ensure it does not run dry of brake fluid.

The Pump and Hold Method with one person, this uses either a "one man" bleeder tool, or a special one way bleeder valve. These methods have the disadvantage that air can be sucked back into the system via the bleeder valve threads, this can be alleviated somewhat by using Teflon tape on the threads. This method is not recommended, with only one person there is no way to verify if the fluid coming out of the bleeder valve has any air present, maybe from a bad master cylinder or incomplete bleeding.

The Vacuum Method, the master cylinder is topped up and the cover is left loose, a specialized vacuum pump is attached to the bleeder valve, this 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.

The 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, the bleeder valves are opened one at a time until the fluid is clear of air. One advantage to this system is the pump reservoir holds enough fluid and avoids running dry, this method is used in most professional garages.

The Gravity Method is the most simple, loosen the bleeder valve and the cap of the master cylinder reservoir, the fluid will drip slowly by the force of gravity. The bleeder valve is designed to be the top point of the fluid in the caliper. The air will not enter the caliper downward from the physics law.

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

If you are bleeding the brakes because the master cylinder is being replaced, the master cylinder is usually "bench bled" before installation, securing the cylinder on a bench, fill it with fluid, connect the 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 is the preferred way to carry this out.

Please Note: The following description of brake bleeding techniques is intended to provide an overview of common methods, it is NOT to be used as specific instruction. The actual procedure varies from vehicle-vehicle and the manufacturer's shop manuals should be used, if unsure contact a qualified professional.