The old way always works best.
Strip all the fittings from it, plug the holes soundly, put 1/2 a pint of paraffin or white spirit in there, and two handfulls of old small nuts and bolts, or small sharp stone chippings, then shake and shake til its all loosened. A small child is often usefull here.
Better still if you got a lathe, put in in the lathe on ultra slow and rotate for a few hours with the stones inside so they slide around, -- not stuck to the sides by centrifugal force, -- thats what I did when I was a mechanic.
Be sure you clean it out thoroughly after.