Losing prime

im well versed with the 2ks. so let me see if i can be of some help.
First of all, the secondary filter is the one mounted to the block. Thats where your primer will be. so im thinking you removed the primary. Whats the reason you removed the primary? The primary is the first line of defense to grab any major debris if any and to grab any water that may be in the tank or the lines. The secondary which is a 2 micron filter from cat is the last line so the injectors dont get clogged. If you were to hook a fuel gauge to your secondary housing, and let it sit over night, its not gonna hold any readable psi. For whatever reason they dont but they hold enough fuel psi to where truck fires up first turnover.
So in the secondary housing, you changed the check valve/regulator already, and you already put a new fuel pump. so lets rule those out. I wouldnt nother with pickup line from the fuel tank. But you can check those by putting a air fitting on top of fuel tank and putting about 1 psi of air to the tank. Thats how you find a fuel leak the quick and easy way from the supply to the pump. The return can have leaks and not effect anything. So next step before you suspect injector leak, is pull your primer plunger off the secondary and check the valves in the plunger assembly or replace it.
Im assuming the plunger is loose when you have to prime it? and not hard to push in when you go to prime it when it wont start. Instead of trying to start it tomorrow morning, pop the hood and push the primer and veryify it is not hard to push in.

 

I have an extra one cat plunger I can send you if you want to try it. It’s only a year old. Other than that maybe try a new fuel filter also and put pressure to the fuel tank to see if suction line is leaking and allowing air in system. When you put shop air to tank, you will see fuel spraying out from where ever leak is

 

I had same starting issue before with a bad plunger. The check valves looked good but somehow it was causing prime issues. So yeah that would be a good start. Let me know if you still need mine. Start with simplistic things first