master cylinder

The master cylinder is the pump that turns the brake pedal's push into hydraulic pressure. Press the pedal, and a piston inside the cylinder forces brake fluid down the lines to each wheel, where that pressure squeezes the pads against the disc or pushes the shoes against the drum.

Because brake fluid cannot be compressed, the pressure arrives at all four corners at once, and the cylinder's bore sets how much force it produces for a given push. Fluid is stored in the reservoir sitting on top, and most cars use two internal circuits so that front and rear still have some braking if one circuit loses pressure.

A spongy pedal, a pedal that sinks slowly to the floor, or fluid found under the cylinder or running down the brake booster all point to a leaking master cylinder, the one brake fault a question expects you to treat as urgent.