If you can, I'd pull the copper line all the way out inspect it for any spots where it's rubbed thin against clamps or the frame from vibration. That's the most common issue I've come across for soft metal lines.
As for the varnish inside the line a lot of that may disolve with the detergent in todays fuels, and like briank mentioned an inline filter will catch any chunks of goop that may break loose. I used the glass bowl type with a 1" long union tube between it and my fuel pump.


1956 GMC 370 dump " 'Tater "
1970 VW Volksrod "the Black Bomber"
2007 Chevy Avalanche
2020 Jeep Cherokee Trailhawk,
2005 Harley Davidson FLHTCUI

I dig all cars, old and new, whether they were hammered out of American iron, German steel, or Japanese tin cans. Being unable to appreciate them all is missing out on a world of great things.
But thats just MY opinion!
:P