A friend of mine had braided SS line from tank to engine, with an OEM style in-tank electric fuel pump. Replaced the pump, but chasing the problem like yours he found that the old SS line (25 to 30 years old) was totally eaten up inside by ethanol fuel and was developing pinhole leaks. If it's an older build with flexible line you might want to check, or simply replace with hard line or ethanol friendly SS line.