Pressure In Fuel Tank When Removing Gas Cap

[LuvNLife]

I sure could use some advice on this one...

'87 GMC square, 305/700R converted to carb. Running a Holley Red with a filter inline before the pump and a regulator at the carb. Because of the low fuel pressure of the pump, it's a returnless system. I just got it back from the body shop today - the paint looks great, for what it's worth.

While it was in the body shop, and the bed was off, I had them install a between-the-rails 19 gallon fuel tank, with the fill now in the bed. It starts and runs great, but I only drove it about 1 mile to the shop of a friend of mine who'll tuck it inside for me. On the way there, I stopped for gas because the body shop mechanic forgot to install the new fuel gauge. When I unscrewed the gas cap (threaded, with an O-ring seal), I smelled raw gas, and when I got it almost completely out, a real blast of air came out; no fuel, just a hell of a lot of air. It's almost like the tank is pressurized.

Any thoughts on where I could start troubleshooting this, or is it nothing to be concerned about?

Jerry