You are simply pushing fluid through the system till you have a physical barrier to stop the fluid flow and stop the pedal.
On front disc brakes there are no return springs to pull the caliper pistons back off the rotors when you let off the brake. Instead all they use is the rubber seal around the piston. They are designed to "roll" when the caliper applies the brakes, and then the seal "unrolls" pulling the piston back just a tiny bit. They will normally lightly rub the rotor they are so close. So there is hardly any travel in the front system before a "barrier" physically stops the caliper pistons.
The rear brakes are a different story. The shoes are pulled back by strong springs. As the shoes wear, unlike the front, unless the shoes are adjusted manually or automatically, the space between the shoes and the drums grows larger and larger. The springs keep pulling the wheel cylinder pistons back all the way each time the brake pedal is released. So that is where all the pedal travel comes from, all that space between the shoes and the drums. Unless you get back there and get rid of the space by adjusting the shoes out.
