[bobcowan]

When they get really stuck, it's usually the big rubber gasket that's the problem. If you keep twisting on the can, it's probably going to tear, and then you're really hosed.

Get a thin narrow blase, like a putty knife or a gasket scraper. Try and work it in to loosen the rubber gasket seal. Be carefull. If you damage the mounting pad on the block it will never seal properly.