Stihl 070 air in fuel line

[macstrange]

I have this 070 that I use for milling. It's been running great until recently. I keep seeing air in the fuel line. I have bought about 3 new fuel lines and only 1 of them works reasonably well. I don't want to keep buying fuel lines if they are not going to work. I tried using o-ring grease but that doesn't seem to work. Is there anything that I can use to make it air tight?
Thanks in advance for any suggestions.