The return of the piston is called the "roll back effect"the seal is deformed when the brake is applied by the piston movement,when brake released the seal returns to its natural form and returns piston to original position. On some calipers mainly Rover the edge of the grove housing is machined to allow the seal to deflect further than normal to improve the "pull of action"of the piston,this is called "reduced Nip Seal",Problem with this there can be slight leakage which can be mistaken for a faulty seal. HTH Regards Tony