Brake Fluid: 

The Lifeblood of the Brake System:
Brake fluid is often overlooked as one of the essential components of an automobile's braking system. Although not monitored or replaced to the extent of motor oil or transmission fluid, brake fluid is nonetheless critical to the performance - and stopping - of the vehicle.  Its neglect could easily lead to costly service bills into the hundreds or thousands of dollars. Brake fluid is probably more susceptible to contamination than any of the vehicle's vital fluids. This is a fact which makes attention to the fluid even more critical. DOT 3 and DOT 4 brake fluids, the most commonly used fluids here are "hygroscopic," meaning that they absorb water. So great is their tendency to absorb water, in fact, that leaving the lid off of a can of fresh brake fluid can cause to pickup moisture. The fluid will actually pull water molecules out of the atmosphere. Moisture can enter the system when the master cylinder cap is removed to check on the fluid.  That's why most master cylinders are transparent so levels can be checked without opening the lid. Moisture can even enter by seeping through microscopic pores in rubber seals and hoses.

The Trouble with Water:
Water contamination is harmful to the fluid and to the braking system.  Because water boils at a lower temperature (212 F) than brake fluid (470F), it causes the boiling point of brake fluid to drop. This leads to fluid boil and brake fade during hard use. Boiling brake fluid results in "vapor lock," for a soft, spongy pedal at best and complete brake failure at worst. Moisture contamination also contributes to internal corrosion in the calipers, wheel cylinders, and steel brake lines. The regular replacement of contaminated brake fluid restores fluid heat and corrosion resistance. How much water is too much water? If a brake fluid leaves the factory with a "dry" boiling point of 470 F, and absorbs about 1.5 percent water in its first year of used, it's boiling point will drop to about 350 F, and after two years 295 F. The contamination is about 2.5 to 3 percent at this point, making many fluids potentially dangerous because they can no longer handle the heat generated by braking. In a recent study of 1,720 vehicles, nearly 20 percent were found to have brake fluid contaminated by 5 percent or more of water. There are 97 million cars over three years old on our roads. Half of these still have the original brake fluid & many are eight to ten years old. Today's cars generate more heat around the brakes.  In emergency test stops it isn't unusual to have temperatures of 500 F or higher at the front wheels. This temperature is much higher than the average three-year-old brake fluid's boiling point. Contaminated fluid is damaging to a braking system, particularly to more expensive ABS systems. You would probably prefer to pay a relatively modest $75 to $125 charge for a necessary brake fluid change than $1200 for a new ABS hydraulic modulator. Therefore one can appreciated that a brake fluid change can help ensure safety and save money.