Brake fluid is a hydraulic fluid that is used in automotive braking systems. The most often used brake fluids on the market today are glycol-ether based. However, mineral oil based brake fluid is available as well.
The brake fluids used in the United States must meet certain requirements set by government organizations, such as The Department of Transportation (DOT). The DOT has its own classifications for brake fluid, based on the Society of Automotive Engineers (SAE). The DOT follows the SAE recommendations but also keeps in context local details, such as the temperature and humidity. With this in mind, it makes sense to hear the terms "DOT 3" and "DOT 4" when referencing brake fluid for a vehicle. Different types of brake fluid should not be mixed. The vehicle’s user manual will specify which type of brake fluid should be used.
The usual measurement of how long something lasts on a vehicle is measured in miles traveled. So when determining how often brake fluid should be changed, it is important to know how many miles have been traveled since the last brake fluid change. However, there is no standard that can be applied to all vehicles. Some companies may suggest as few as 20,000 miles while some will suggest that the vehicle may travel up to 150,000 miles before changing the brake fluid. Owners should consult the vehicle’s user manual, or a licensed mechanic, to determine if the brake fluid needs to be changed.
Brake fluid may need to be changed outside of the regular maintenance schedule, such as if moisture has gotten into the braking system, or if the brakes become sloppy or lock up. A licensed mechanic, such as one from YourMechanic, can help correctly diagnose braking issues, at the convenience of your home or office.