Figure out which piston(s) are stuck and spray hell out of it with ....whatever.....I like Sea Foam Deep Creep.....let it sit a while. Then find a piece of 1/4" plate small enough to fit on top of the pistons and put it on top if the pistons one at a time and blast on it with an air chisel. That will give it some serious vibratory action and work the penetrating oil into all the crevasses.