Originally Posted by
DaveNur
"Brake fluid resides in a sealed, air-tight reservoir on top of the master cylinder. As the brake fluid level drops, air moves in to fill the void. Wait a minute -- didn't we just say that the system was air-tight? Well, unfortunately, air does get into the reservoir over time and it eventually makes its way into the rest of the braking system -- including your brake lines. Every time you open the cap to check your brake fluid level, you're letting air into the system. When the fluid is at the correct level, air isn't much of a factor; however, the more worn your brake pads are, the larger the void grows. That's one reason why it is so important to keep the fluid at the proper level.
*Water is another factor that can cause brake issues. Water can actually cause air to accumulate in your brake lines. How so, you ask? Well, brake fluid is hygroscopic -- it absorbs and retains water. The problem arises when the fluid has absorbed as much water as it possibly can -- it becomes saturated. Brake fluid is designed to have an extremely high boiling point; however, if you subject your vehicle to excessive braking, like maybe a trip through the mountains, you can easily cause the brake fluid to heat up to this temperature or beyond. When the brake fluid boils -- especially when there is excessive water in the system -- steam is a by-product. The braking system ultimately compresses this steam and turns it into water. The air separates from the water and before you know it, you have large pockets of air in your brake lines." - - How Stuff Works.
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