A Cessna 172, for example, uses a single-disc system. When you press on the toe brake, a master cylinder that’s attached to the brake pedal pushes hydraulic fluid through hoses and rigid lines to a brake-unit housing attached to the landing gear strut. A piston within the housing responds to the pressure by pushing against a lining, which then pushes against a brake disc, which in turn pushes against a stationary lining. The equal friction on both sides of the disc is what slows and stops the airplane. If an airplane is equipped with pilot and co-pilot toe brakes, there will be a master cylinder and reservoir for each brake—and also one for the parking brake.