Losing fuel pressure on new setup

the easiest thing to do is try another fuel pump or put in a mechanical , replace the filter , bypass the regulator. obviously the pump you are using is not working or something is restricting the fuel. there should be a constant 5.5 -6 psi to the carb, if the pump isn't delivering that , then you need to try something different. I have a tank vent , I plug it off in the winter for storage. I forget to unplug it sometimes , I run and drive the car , I never lose fuel pressure before I remember to unplug it. I do not have a vented cap. your gasoline is not going to cause a loss of fuel pressure. maybe you have a rubber line collapsing. just because parts are "new" does not mean they are "good". it only takes a few minutes to try another pump and run it from the tank to the carb with just a filter