Changing the many fluids in a vehicle is always a change for the better. Dirty engine oil, transmission fluid or anti-freeze are bad news for a car. But what about brake fluid? Many motorists know that this fluid should be topped off, but changed?

According to the Car Care Council brake fluid in the typical vehicle can become contaminated in two years or less. This is because the fluid absorbs moisture, which works its way through the hydraulic system. Under heavy braking conditions, such as those encountered in mountainous or hilly driving or when towing a trailer, moisture in the overheated fluid vaporizes (boiling point of water is lower than that of brake fluid) and braking efficiency is reduced.

"Even under normal driving conditions this condition can develop if the brake fluid is seriously contaminated" says Rich White, spokesperson for the Car Care Council. "Not only is the fluid vulnerable to vaporizing, it also can freeze.

Brake fluid must maintain a stable viscosity throughout its operating temperature range. If it's too thick or too thin, braking action is impaired. Beyond the vaporization hazard, moisture creates an additional problem for owners of vehicles equipped with anti-lock braking (ABS) systems. Rusted and corroded ABS components are very expensive to replace.

How does a car owner know when to have fluid changed? The Council recommends replacement every two years or 24,000 miles.

"Certainly it should be included with brake pad or shoe replacement," White emphasizes. "In between, as a preventive measure, a professional brake technician should check the condition of the fluid with an accurate fluid test safety meter, which is inserted into the master cylinder reservoir to record the fluid's boiling point.