I just changed out all O-rings to viton, incl. del. valves, as well as a couple lines. I have air bubbles in the first line from the pre-heater/thermostat to the pre-filter. I also cut back the black rubber hose coming from the ridgid metal line going to the pre-heater. I did this to get a fresh area of rubber to clamp, before it was loose.
I believe that I'm getting the air bubbles from back further, maybe at the tank? Is this possible? I plan on changing the tank strainer soon anyway.
Also note ;Only bubbles in ,no fuel out now since the O-ring changes.