Michiel if the piston has no extraction groove, I can only imagine it should have a circlip with eyes or one with the separate tang to get hold of. No help I know I guess it's a case of just digging with a small sharp screwdriver and small pointed prodger. If you don't need the piston? An angle grinder with thin cutting disc comes to mind. Cut through the end of the gudgion pin hole untill you're through the circlip. Ron