I am trying to replace the pump valve on my Petromax. I've unscrewed the valve and it is loose and wobbling in it's housing, but it will not come out! I've tried banging the tank, holding it upside down while flicking the valve with a screwdriver but it's not dropping out. I can't see anything on the replacement valve that would explain why it is sticking. It's very puzzling - has anyone else had this problem or does any one have any suggestions? Many thanks.