On a healthy fuel supply system there should always be some. When you say it made no difference was there fuel coming out the return line in your test?

I suspect you mean the engine acted the same as when the lines where connected though.

The reason for the return flows presence . . The average old relief valve if still regulating opens at around 10-12 pounds fuel pressure. The lift pump in good shape on your model with fairly decent fuel filters provides at least twenty pounds and usually more fuel pressure.

The volume of fuel at the elevated pressure beyond the 10-12 pounds is returned to the tank. Some relief valve springs have aged to death and are opening at far too low a pressure.

The manufactures reason for this setup is to make sure there is always a stable fuel supply at a desired pressure available in the injection pump. Plus incidentally or intentionally core heat generated in the injection pump is reduced by the constant fuel circulation.