Originally Posted By: Warstud
Over time the moisture becomes trapped in the internal sections of the brake system. When this water reaches 212º F the water turns to steam. Many times air in the brake system is a result of water that has turned to steam. The build up of steam will create air pressure in the system, sometimes to the point that enough pressure is created to push caliper pistons into the brake pad. This will create brake drag as the rotor and pads make contact and can also create more heat in the system.
Not quite. If the water were held in suspension as it would be in oil, this would be true. Water is miscible in brake fluid (DOT5 excepted) and as such the boiling point for the mixture is somewhere between the boiling point of clean water and the boiling boint of the brake fluid. The more water you've got in there the cooler it can be and boil so less water is better but in any case the critical temperature will be higher than 212 degrees F. One way or the other, if you do manage to get your water to boil, the steam won't create air. Any good alchemist can tell you that transmuting matter like that isn't quite so easy. The steam will eventually condense back into liquid form when it cools.
The steam also won't do much as far as generating pressure and activating the brakes with the pedal released. With the pedal released it will just push fluid back into the reservoir through exactly the same path that allows fluid to enter the master as pads wear. The opposite problem, that of not generating pressure, is more serious and is real. Because steam is compressible, fuid displaced in the master does not all go to displacing fluid at the slaves because it also has to compensate for the loss of volume as the steam compresses under pressure. This means that with steam present you get a soft pedal feel and inadequate brake apply.
