the average motorist who drives 10,000 to 15,000 miles a year uses ... the brake fluid in the average vehicle may contain as much as two percent water. After 18 months, the level of contamination can be as high as three percent. ... Brand new DOT 3 brake fluid must have a dry (no moisture) boiling point of at ...
e.g.,Chevrolet calls for a brake fluid change on most models every 45,000 miles, but Honda says to do it every three years regardless of the vehicle’s mileage. Three years is also the recommended interval for most Volkswagens, but Mercedes-Benz vehicles typically call for fresh fluid every two years or 20,000 miles. In contrast, on the Ford Escape, Hyundai Elantra, Toyota Camry and other models from those manufacturers, there are no recommendations for replacing the brake fluid, only instructions to inspect it periodically.
This leaves it up to the owner to consult what the manufacturer says in their car’s maintenance schedule and rely on the advice of a trusted repair shop.
Brake fluid lives in a sealed system and can survive for years, but moisture from the surrounding air can work its way in through hoses and other parts of the brake system. If your brake fluid has become dirty or contaminated, it can change how your brake system operates — brake pedal feel can be affected, as can heat dissipation in repeated stops. Water in the brake lines lowers the boiling point of the fluid, so stopping ability can diminish in hard stops as heat in the system increases. In addition, over time the moisture can cause internal corrosion in the brake lines, calipers, the master cylinder and other components.