To understand brake fluid ratings we have to touch on what brake fluid does. Brake fluid is a type of hydraulic fluid that is used to transfer force from a brake pedal or brake lever to the brake caliper or wheel cylinder, which converts it to pressure. When the caliper or wheel cylinder exerts pressure on the brake pad and rotor combination, or the brake shoe and drum combination, the friction slows the vehicle down. Brakes convert kinetic energy into heat, and that’s where brake fluid ratings come in. The DOT ratings for brake fluid represent their boiling points when dry or wet. Brake fluid contains glycol which attracts water. Over time it is normal for brake fluid to absorb a certain level of moisture, so brake fluid ratings include a “wet” boiling point number along with a “dry” boiling point number.
Most applications call for a DOT3 (401F dry/284F wet) or DOT4 (446F dry/311F wet) fluid, which is considered a higher performance fluid since it can handle more heat. While both fluids are glycol ether based, DOT4 contains a certain amount of borate ester which allows it to handle more heat.