What does master cylinder do?

The master cylinder is the heart of a vehicle's braking system. It's a hydraulic pump that converts the force you apply to the brake pedal into hydraulic pressure. This pressure is then transmitted through brake lines to the wheel cylinders (or calipers in disc brakes) at each wheel, causing the brakes to engage. In essence, it amplifies the relatively small force you exert on the pedal into the much larger force needed to stop a vehicle.