I'm debating this same issue. Here's what I'm confused about.

If my header tank is full--which it would be all the time unless I was minutes from fuel exhaustion--then I have continuous gravity pressure trying to make the fuel flow from the tanks, to the header, and then to the engine. the fuel doesn't care if it travels through the bottom fitting or the top fitting of the header. Having said that, there would be constant back pressure to the fuel return line. But the fuel return line is supposed to dump vapor (and I presume small amounts of fuel, into the header--which it cannot do because it is being met with the fuel coming FROM the header. Now I'm guessing the fuel pump provides extra pressure, but I've seen the little jet inside the banjo bolt through which the fuel and vapor are supposed to travel, and I can't imagine there is much pressure left after that.

Any ideas?