Right the Piston: Do use a G clamp. Don't use it on it's own. However just try getting a screwdriver accross the piston head and slowly push it back.
If like shadi's and it's jammed you needto use a hammer handle (or equal size - metal bar etc) and cover the whole piston "face". then clamp it and if need be use a pair of pilers to twist it back otherwise your gonna get sore hands.
Main reason for this is it doesn't offset the piston when it's being pushed back.
