English is not my native language, but I try to explain. The brake acts as a hydraulic pump. The brake pad is forced against the disc by a piston. Between the piston and the housing is a sealing, O- shaped. When the piston moves towards the disc, there is deformation of this ring. It sticks at the inside to the piston and at the outside to the housing. When the brake lever is released, the piston is retracted (only very little!) by the O ring because the ring tends to have the original shape. When the brake pads worn out the piston slids inside the sealing to a new position. A simple drawing would explain it more easy, but I don't have.
Its been a pretty long time since my job as motorcycle engineer, but as far as I know the basics are the same or??