Gas Tank Vacuum

[PbFut]

I've owned the truck (72 GMC C20) less than a year. I've had it apart and not drivable more than running (by choice). Recently back on the road, I notice the gas tank has a pretty strong vacuum on it when I open the cap. I don't recall this happening previously but the old cap was not sealing. There is a new cap from Brothers on the filler neck and says on the sticker it is a sealed cap, watch for splash or something to that effect.
Is the cap defective or this normal? Shouldn't the cap allow air in, but not out? It's pronounced enough that the tank sounds like it is deforming as I'll hear the gas slosh and sounds like the tank wall pops out from the deflection from vacuum. Or at the station after diving a a half hour or so, I open the cap and air rushes in and you can here the tank go back to shape. That seems excessive.
Wouldn't it also effect fuel delivery?