One helpful member mentioned putting a piece of clear fuel line in the fuel return. That way if say air entering is a variable or system drain back because of worn lift pump valves you will see air in the fuel.

There should be nothing more than solid fuel in that line at all times. Air has more effect on idle than at any other operational point on these engines. If you have no overflow into that line also fuel pressure may be low. If the valve made a difference when installed it probably has good enough fuel pressure. It should still overflow though and I assume the major fuel filter is not ancient.