I have had great luck using copper washers to seal inverted flare fittings. I keep them in my tool box in several sizes to use whenever I have a flare fitting that won't seal. They work on fuel lines, brake lines, transmission cooling lines, whatever. You can order them online from Grainger, see link. They are made specifically for inverted flare fittings.
https://www.grainger.com/product/5WR...E&gclsrc=aw.ds