If the piston has extended itself and you cannot pry it back into its bore, you will have to take the caliper off and use a C-clamp, as others have suggested. You cannot take the brake hose off the caliper until you remove the brake hose from the connection at the end of the brake line. This connection is not too difficult. Just use two wrenches. One wrench goes on the brake line and the other goes at the end of the tubing. In fact, if I remember, there is a steel plate that prevents rotation of the tubing. Once you have separated this connection, you can take the caliper off the vehicle and work on it. Much easier than trying to deal with it on the vehicle. When using a C-clamp, be very careful to ensure that the C-clamp is providing even force against the piston and is not trying to cock the piston. The clearance between the piston and the bore is very small and, if the piston is forced into the bore while is it slightly cocked, you are hosed