Gas cap problem!!

[Jaysonb]

I just got done finishing the paint and putting together the cab and front end of my 71 C20. One of the many new parts I got was a nice new gas cap, I got it from GMC Pauls when I ordered the rubber kit from him. I was putting the new interior in it today and walked around the back of the truck for something, and the back of the cab below the rear glass was totally bowed out, I mean enough to leave crease marks. It took me a few minutes to figure out that the gas tank pushed out the back of the cab!!! I still have the box that the cap came in, and it says vented on the box, but must not be. Anyone heard of such a thing? I checked out the tank real good, and it looks ok, and isn't leaking, but man was there allot of pressure in there. The good news is that the creases on the back of the cab wont be noticeable with the box on.