Picture a brake pedal in your mind. Delve into the relative darkness of that imaginary footwell and push that pedal with your mind foot. In most cars the pedal motion directly pushes a rod (known, funnily enough, as a pushrod) into one end of a sealed Brake Wheel Cylinder filled with fluid, two pistons to displace the fluid, and springs to push back against the main force and return the brake pedal to its resting position when released.