Well I got impatient. Pried it off with a crowbar, thoroughly broke the locking mechanism to ensure noone could lock it again, then held the cap down with a piece of random bracketry.
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My cap is in the same situation. Loose but won’t release. I’ve been working it back and forth with a pair of channel locks. No joy. But prying it off seems to stress the tank itself and I don’t want to have to replace it too. How did you pry yours off?