Brake fluid, which can become contaminated, should be replaced periodically as a safety measure. Six fluids that require periodic replenishment or replacement in most vehicles are the engine oil, transmission fluid, anti-freeze/coolant, windshield washer solvent and fuel. Number six, the one most often neglected, is brake fluid. You’ve known about topping off brake fluid, you may say, but changing it? 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 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 circumstances, this condition can develop if the brake fluid is seriously contaminated. Not only is the contaminated 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. It should be included with brake pad or shoe replacement, the Council 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.