Brake fluid is hydraulic fluid that has specific properties. The fluid is used to transfer force while under pressure through hydraulic lines to the wheel braking system.
Braking applications produce heat so the fluid used must have both a high boiling point to remain effective and a low freezing point so as not to freeze or thicken in cold conditions.
Brake fluid is hygroscopic, which means it absorbs water from the atmosphere. This will gradually reduce its boiling point, so the fluid should be changed periodically to remove water and other contaminants and to ensure the continued effectiveness of the braking system.
The properties of different types of brake fluids are tested for many different characteristics such as ph value, viscosity, resistance to oxidation, and stability, and graded against compliance standards set by the United States Department of Transportation (DOT).
Brake fluid DOT specifications:
Even if they have similar base composition, fluids with different DOT ratings must not be mixed.