1997 GMC C1500 Vaccum/Pressure released when removing gas c

I hear a lot of hissing when I remove the gas cap on my truck. I just replaced all fuel lines (supply,return,vent), fuel tank, complete fuel pump, filter, filler neck, and gas cap.

The truck runs fine, there's no check engine light.

I checked the vapor line to the canister with vaccum/presure pump engine not running. it held both vaccum and pressure.

my concern is that the tank may implode or colapse if its not vented properly.

I don't ever remember any hissing from the tank before. I've owned the truck since it was new.

Thanks