When I push back a caliper piston, I pop the top off the master and leave it on but loose. If the pads were extended and someone topped off the fluid, you're also fighting against the fluid compressing in the master reservoir.
To push the pads back, I place the old pad over the piston and use a 4" C clamp, this allows it to push back evenly and not possibly cock the piston in the bore. If it gives me a fight, I either rebuild or buy a new caliper.
Bob