Unless you change something else in the system, you need to open that line. Originally, there was a ball check valve in the engine bay end which was supposed to help keep liquid behind it. Download the manual I posted and do some reading.
I would say you have several choices
1--open the line, run a hose/ line from it as high as you can and safely out into say, the fender area to vent fumes, but don't run it too low, or you can create a siphon.
2--Modify the filler so it's vented like the older in the drawings posted in those threads
3--Find a vented cap. The problem with vented caps is that you run the risk of having fuel spill on the rear quarter panel.