How is the gas tank supposed to vent on a '68

[toms68cst]

A lot of times when I go to fill my truck in the warmer weather, the gas tank will depressurize (I THINK??) considerably when I take the gas cap off. It seems like it is most apt to happen when it's been cool in the morning and hot in the afternoon. I notice it more when the tank is less than half full. I don't drive my truck a whole lot so I don't fill it sometimes for several weeks.

I checked out the original gas cap and there seems to be a little check valve in it that lets air into the tank but not out.

I think the 72's at least were vented differently.

Is this normal? It has never really caused any issues with gas supply to the engine. I just don't want my tank to burst or collapse.