what is a brake master cylinder

The Brake Master Cylinder is the first component in a vehicle`s braking system, activated by depressing the brake pedal. 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. The Master Cylinder is designed to last the life of the car, but like brake calipers, sometimes it can spring leaks or otherwise fail, and it must be rebuilt or replaced. The Master Cylinder is topped by the brake-fluid reservoir, which must be kept full.