What is the Difference Between DOT3, DOT4 and DOT5 Brake Fluid?
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.
DOT5 is a silicone-based fluid that was developed for use where moisture or water was almost certain to be a factor in the braking system, such as military applications. While DOT5 has a higher boiling point (500F dry/356F wet) than DOT3 or DOT4, DOT5 exhibits more compressibility than glycol ether brake fluid. There are DOT4 fluids with boiling points that exceed DOT5 so there’s really no need to consider using DOT5 for on-road use.
DOT3 and DOT 4 cannot be mixed with DOT5 fluid. They are incompatible and if mixed, brake failure may occur. Moreover, there are serious compatibility issues when using DOT5 in a system that was engineered to use DOT3 or DOT4. Also, users should take care not to mix DOT3 and DOT4 fluids as there may be brake system compatibility issues. Always consult your owners manual and use the type of fluid specified for your vehicle.
One last thing to know when discussing brake fluid is federal standards for brake fluid only dictate boiling points, not fluid composition, so there is no “standard” brake fluid formula.