Losing prime overnight/fuel pressure problem

[Skman]

I've been trying to figure out an issue with my isx2250 the last couple days.
The other day after the truck sat overnight for about 10hrs...my driver went to start it and it would only turn over...no fire.
I got down to take a look and my PACCAR fuel/water separator was almost empty.
We tried priming it with the lift pump many times and it wouldn't start. The lift pump was changed a couple months ago because it was weak so I was pretty sure it wasn't the problem.
Eventually, I had to use a little starting fluid and got it to fire and it started running and sounded good after about 10-15 seconds.
Driver went out to work and he said that it had hardly any power on even small hills.

Today I went over and wouldn't start again.
Here's what I did today:
1. Changed both filters. PACCAR bowl filled up instead of being almost empty like before.
2. Bypassed the fuel water separator and primed from a Jerry can.
I can hear fuel running to tank during the priming process and after I shut off the key.
Still wouldn't start after priming about 10 times before trying to start.
3. Cracked the nut on the relief valve (about 3 months old) and it started dripping/trickling out pretty good. Probably a cup or two of diesel came out.
I didn't have another relief valve handy so I put it back together.
4. Tried a fuel decay test in InSite. Measured would NOT go past 13,000psi. But when I turned off key....it would hold within a couple hundred PSI.
5. Used a little ether again to start it and ran cylinder tests. Each one made a noticeable difference when cut-out.

Any ideas what to do next?
I thought maybe a bad relief valve on the rail but even though it doesn't build to 29,000 on the test...it does hold at 13,000?