To be clear, did you change both filters and then try to start it?
I understand you have it running which is a very serious relief for sure.
I learned the hard way to NOT change both at the same time. Change one, start it and run two minutes, change the other.
I know the expert here says to not pre-fill them but that large lift from the tank to the engine may be the issue on some of our setups.
My filters actually show on the sides how to pre-fill. I have not had an issue in ten years now since pre-filling and starting after each filter change.
I saw another thread with same issue not long ago and they were going to add an inline pump. I suspect that would really help.
I think a simple check valve could do the trick as well.
Certified Senior Electronic Technician, Telecommunications Engineer, Telecommunications repair Service Center Owner, Original owner HR 2008