Fuel Tank Pressurizing 1973 F-250

[LHCBOY]

While driving I get enough pressure in the tank to make the cap gasket whistle as vapor escapes. The truck has no evap system. There is a filler tube and vent on one side and the line going out to the pump on the other. This is the middle tank and lines from rear tank are plugged off. Question one is: Where is the pressure coming from? Question two is what to do about it? Is there a cap that vents both ways? Appreciate any advice.