Help! Pressure Build-up In Gas Tank

[pjmoreland]

I have a 67 Chevy truck that has a problem where sometimes I get a massive build-up of pressure in the gas tank. When I take the gas cap off, air and gas come spraying out. The biggest problem with this is that sometimes the pressure forces the float valve in my carburetor fuel bowl to get forced open, causing gas to dump into the carburetor, making my truck die. This problem usually occurs on days when the morning is cold, and then it heats up quite a bit in the afternoon.

The problem seems to be caused by the gas cap. The gas cap has a check valve in it that lets air in as the gas tank empties during normal operation, but the check valve does not let pressure out. I got so fed up with this, that I drilled the check valve out, but now if I go around a corner, gas leaks down the side of my cab. At least this is better than having the engine die because it is getting massively flooded. It's not a very good solution though.

When I bought the truck 12 years ago, the original owner told me that the truck required a vented gas cap. The gas cap that came with the truck wore out several years ago, and I have not been able to find a vented gas cap anywhere since then. I'm wondering if he EPA has banned vented gas caps, or something like that.

Has anybody run into this problem? If so, what have you done to fix it? Does anybody know if vented gas caps are available anywhere?

Thanks for the help.