Just be aware a lot of dowty seals are NOT fuel compatible, the most compatible are NITRILE.
Neoprene(black), EPDM, Tan Shotblast, Insertion rubber, Neoprene(white/food grade), Silicone, and lastly Klingerseal C-4324, should not be used with fuel, most will seal on a temporary basis, but not long term.
The problem is the copper often is NOT new, but years old, even when freshly bought, if heated to cherry red and allowed to cool will seal perfectly if not tightened overly tight.
tabetha