In automotive engineering, the master cylinder is a control device that converts force into hydraulic pressure. This device controls slave cylinders located at the other end of the hydraulic brake system. In brake systems, the operated devices are cylinders inside brake calipers and/or drum brakes; these cylinders may be called wheel cylinders or slave cylinders, and they push the brake pads towards a surface that rotates with the wheel until the stationary brake pad(s) create friction against that rotating surface .