DD15 wont fire after fuel filters changed.

Are you 100,% sure you have the plastic ring on correctly and it is properly tight on the Davco? Those can be a PITA to get on correctly .. but even the slightest crack on the plastic ring will let it suck air and won't prime
Just say'n​

 

There is a rubber grommet on the filter that is supposed to come out with the filter when changed. If it stuck and stayed behind, then two is not good.

 

Maybe some pictures would help, someone has to be familiar with the system

 

Do you have fuel shutoff valves? My Cascadia had the long handles just under the drivers door that operated the valves mounted above the transmission so you could control fuel flow from either tank or both.
Had a Century where the weight of those control rods to the valves vibrated wear in the valve stems and allowed air in and I lost prime. Took a whole day to find that one.
We discovered it when we fed fuel from a container directly to the inlet of the fuel pump. Everything ran fine, then it would lose prime when we reconnected to the shutoff valves.

 

I had a hair line crack on my Davco o ring. I would prime it and if I didn't start the truck in 2 minutes, it would lose prime. It was a cut that didn't go thru the entire o-ring.

 

I had my mechanic here in the office for another issue and I asked him about this. He said because you went through all this fuel, it could be a stuck fuel return valve or what is called a "two-stage valve". He said the valve feeds fuel back into the filter unit and then into the tank.

So he suggested you take the return line off the filter bottom, plug it with a cap and try starting it, if it starts, then it is the two-stage valve on the HP pump causing the problem of just not allowing the fuel pressure to build enough for the pump, it should take you 5 minutes to try it.

EDIT - he just texted me that DD diesel knows about this issue and has issued a memo on it some time ago, they recommend replacing the valve but said it can be cleaned - carefully cleaned.