hi, been trying to find a picture of a bike master cylinder to show how it works, plenty of car ones with the floating control piston...complicated. so your going to have to make do with a basic description

the reservoir lets fluid into the cylinder through a hole in the side of the bore, once you push the brake the piston moves along the bore closing the reservoir hole, and creating pressure in the now sealed off cylinder 'pushed' by the main piston seal. there is another seal at the other end of the piston so the fluid does not leak out. once the brake is released the piston moves back, allowing [if nessasary] a topping up of the cylinder

hope this helps bernard