Reading your posting reminded me that I too likely have stainless repro's that is the root to my unique difficultly in ensuring a good seal. And the comments from others posting reminded me of the problems caused by using stainless tubing with braking systems,,,which I have on my 59 Vette with disc brakes which has always been a problem with only one caliper. It is generally believed by many that stainless brake lines can be a real bear and strongly advise usage. No reason why the same could apply to fuel lines. Likely why the copper solution as recommended by earlier postings has worked well, as copper and even steel are far more malleable than stainless and will conform and accommodate idiosyncratic fittings better.

When first installed on my Six Pack, I experienced no leaks. It was only subsequent R&R's where I had problems. The first time seal likely made male and female 'conform' but subsequent R&R's were unable no matter how tightly cinched were unable to undo the original 'set' and adjust to a new set as female and male matings were not exactly the same,,,,even though I would carefully before tightening of ferrules 'wiggle' fuel lines within them about as I tighten them to ensure an exact mating. Yes I do use only the correct flare wrenches.

The next time I have fuel lines removed, I will examine very closely brass connectors and lines.

Using a magnet, I just now checked the lines and ferrules on the theory that some stainless, not all is magnetic just as regular steel. Lines and ferrules both are magnetic so still do not know if steel or magnetic stainless. However as brass is likely softer than either, will be curious to find if any damage preventing proper mating is in the brass fitting itself. Again copper being softer than steel or stainless it could conform to brass and accommodate ok.

The advice on greasing behind the flare on the line is well taken,,,,even when using my teflon tape approach. When using the lubricant approach I do lube this area,,,,but have been negligent in doing so when forced to teflon solution.