high pressure buildup in fuel tank - help!

[hairlesshobo]

I have an '84 C10 that builds up a lot of pressure in the fuel tank. If I fill the tank up and drive it until the tank is almost empty, when i remove the gas cap there is a lot of built up pressure. I know it is pressure and not vacuum because if I loosen the cap quickly, it will literally blow the cap off the fuel neck.

In case it's relevant, this truck was a dual tank setup. The original owner bypassed the secondary tank and selector valve and ran just hoses straight to the driver side tank. I am running a 350 with a 4 barrel, engine driven fuel pump w/ return line to tank.

Anybody have any idea why my tank would be building up pressure?

Thanks in advance

-Steve