Mineral oil
Hydraulic brakes also use brake levers to push fluid through a hose to move pistons in a caliper, thus forcing pads against a braking surface. While hydraulic rim brakes exist, today the hydraulic actuation mechanism is identified mostly with disc brakes. Two types of brake fluid are used today: mineral oil and DOT fluid. Mineral oil is generally inert, while DOT is corrosive to frame paint but has a higher boiling point. Using the wrong fluid can cause seals to swell or become corroded. A hydraulic mechanism is closed and therefore less likely have problems related to contamination at exposed areas. Hydraulic brakes rarely fail, but failure tends to be complete. Hydraulic systems require specialized equipment to repair.