How does the gas cap work?

[TruckintheBoro]

This will sound dumb, but I'm new to trucks. My prior post about why does my carb flood feeds this post. THe mechanics I have been using wondered if there was a chance that pressure had built up between the fuel tank and the carb as temperatures here in North Carolina got up into the 90s... Obviously, with the gas cap in place, as the air above the fuel in the gas tank expands with the heat, pressure does build up. they looked for some sort of venting path to discharge that pressure (they made it sound like that's a normal feature of modern cars/trucks?) but couldn't find one. I definitely find that if I open the gas cap on a hot afternoon, I can hear and feel a good bit of pressure getting released.

There is a pressure valve in the gas cap, but my mechanics were under the impression that that just lets air into the gas tank so that you don't get a negative pressure situation as the fuel level drops, and that it doesn't let pressure back out. Another mechanic suggested one remedy was to drill a tiny hole (1/8") in the bottom of the flange the cap screws onto. Does anyone know how things are supposed to work?