Thanks for the responses, but "just remove the old cap" is much easier said than done. I like the idea of sacrificing the old cap and re-keying a new one, but it's still frozen shut. I've removed the 5 screws which hold the cap in place, but until I turn the key and open the cap, it's still in locked and shut position. Sure, I tried WD40 into the keyhole, even removed the tank and angled it so that the drips would travel toward the latch, but it's been a couple of weeks and that key just isn't going to turn. Maybe I could drill it out?
The cap won't come off because there is a security screw underneath.
Did you try tapping on the cylinder to shock it?
Sometimes downward pressure will take the load off the mechanism and make the key easier to turn.
If all else fails...drilling is the answer.