Venting

NO MOPAR CAPS were vented. (At least the cars we are talking about here)

The ones from 70 (CA) and onwards were "pressure vacuum" in other words a relief valve

Either leave the 1/4" tube "up high" for a vent, or re-create the 69/ earlier vent in the fill tube. From there, earlier, the caps were sealed

Go to MyMopar, download a 69 or earlier manual and a manual for your car. The setup is detailed either in the fuel section or "emissions"