Power-Bleeding Brake Systems

Connected to the brake pedal is a rod that pushes a piston inside the master cylinder. When the brake pedal is pushed down that piston pushes brake fluid through the brake lines. Since this fluid has nowhere to go it pushes pistons inside the brake calipers or wheel cylinders. These pistons push brake pads or shoes against the rotating discs or drums and presto—the vehicle stops. Brake fluid, by virtue of being a non-compressible hydraulic fluid, transfers the pressure between the brake master cylinder and the pistons inside the calipers or wheel cylinders.


Every time work is performed on a brake system it's a good idea to bleed the system of any air that may have entered the system. The reason is simple. Air can be compressed, where as brake fluid cannot. Trapped air in the brake hydraulic system can cause a myriad of problems, from a low or spongy brake pedal feel to outright system failure.


Bleeding the brakes removes any air that may have snuck in while working on the brakes and returns the system to its non-compressible state. While there are many methods to bleeding brakes by far the most efficient and easiest is to pressurize the entire system from the master cylinder reservoir and then bleed normally. This method not only purges the entire system of any trapped air, but also flushes out any possibly contaminated brake fluid and can be performed by one person.

Hygroscopic
Just like every other part of the brake system, brake fluid wears out and needs to be replaced on a regular basis. A more accurate description is that the fluid becomes contaminated with moisture. Over time moisture enters the brake system because brake fluid is highly hygroscopic, which by definition means it readily absorbs moisture from the atmosphere.


Moisture, or water, in the brake fluid creates issues on two levels: both of which can lead to serious problems. Along with being non-compressible brake fluid is manufactured to resist turning from a liquid into a gas - or boiling. If the brake fluid absorbs too much water, its boiling point can drop to a dangerously low level. Water turns from liquid to gas at 212 degrees, a temperature that brakes can rapidly reach. If the brake fluid has reached the point where its boiling point is compromised, it will vaporize when brakes get hot, and can cause brake failure. Ironically the boiling of this moisture also creates vapor, or air, within the brake system, and may be where the air in the lines came from in the first place.