We cut through an old brake master cylinder to give you a look inside. You can see the two reservoirs for the two circuits, front and rear. Holes in the bottom of each reservoir allow fluid down into the system. Below, pistons slide past the fill holes and compress fluid as you press the brakes. Each piston has sweeper seals that match the inside diameter of the brake master cylinder and holes where front and rear brake lines lead to the proportioning valve or to the calipers and/or wheel cylinders.