Brake Master Cylinder is a control device that converts force from a driver's foot into hydraulic pressure. This device controls slave cylinders located at the other end of the hydraulic system. The pedal pushes a piston through this cylinder, forcing brake fluid through brake lines to slave cylinders at each wheel, which in turn drive the pistons that force friction material against the wheel hubs, stopping the wheel.
A reservoir above each master cylinder supplies the master cylinder with enough brake fluid to avoid air from entering the master cylinder. The master cylinder is designed to last the life of the car, but sometimes it can leak and have to replaced.