Gas tank suction

[timspider]

I've gone through posts going back to 2008 about this issue but didn't see the specific solution. So here goes:
Just installed electric fuel pump when mechanical finally failed. But now I notice suction building in the tank when capped. I had the cap loose (forgot to re-tighten)when I first ran the new pump last night. Car ran well except for rough idle.
then would not start this morning, even with cap loose. Tightened cap and tried to re start. Nothing.
My main question is how do I go about reducing that suction in the tank. I have a return line on my carb hooked up via a fuel filter with a spot for it. I'm guessing that line is not breathing properly?
Should there be any back pressure in the tank at all?
Does loosening the cap achieve the same pressure release that the return line is supposed to provide?
My shop manual has very basic info about the tank with none of the details I need.
Thanks in advance.
ps: I was just bragging to my girlfriend last night about fixing the fuel pump so hoping to get this rectified before she notices.