Getting the piston down on changing Barina brake pads?

Im trying to change my mother in laws 2001 Holden/Opel Barina xc 1.4L Brake pads but im having heaps of trouble, iv tried doing everything but i cant seem to get the piston down, does anyone know how i can do this.. Iv changed brake pads before on all types of cars and never had any trouble except on this car