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?

[howzzzit]

You don't have the emissions set up on a C20 do you? My 72 C10 has rhe emissions set up and it uses a non vented cap. Bet you should be using a vented cap. Someone on here will know for sure.

[Steeveedee]

The Evaporation Control System was only used on 1/2T trucks (and cars, ofc) in '72. 3/4T trucks need a vented cap.

[dagnabbitt]

I have the same deal with my 72 GMC 2500, I usually twist the cap open after a long drive to let some air in. Doesn't seem to affect performance in any way and I never thought anything of it but I suppose I might have the wrong cap too.

[Steeveedee]

Having to pull suction isn't going to be doing the fuel pump diaphragm any favors. You can get vapor lock easier, too.