Brake fluid should be changed every 1 or 2 years. Over time, moisture diffuses into the fluid through brake hoses and rubber seals, increasing the water content. If your brake fluid is low, you probably have a problem such as a leak. Leaks and excessive moisture in your brake fluid can both result is a loss of hydraulic pressure and braking ability, so it's as well to check and change your brake fluid periodically. As a general rule, don't mix brake fluids with different DOT ratings.