Fuel Cap Stuck on Truck!

[javadoc]

For some reason, the fuel cap on my truck won't release. It's one of those that you put the key in the center and when you turn it, it retracts the 'locking' fingers, or whatever the technical term for them is.

Any way, it will turn a bit but won't come off. My thought is that possibly the system is sealed and now that the tank is nearly empty, there's a good deal of vacuum which could be holding the cap on. Sound plausible?

Any ideas how to make it come loose? I was thinking of loosening up the fuel gauge sender bolts on the tank and seeing if I could relieve the pressure. Anyone had to do that before?

Thanks for any advice, guys!