If you're losing prime, you're aerating the fuel to the CP3, at the very least.
Fix the leak, then test and move on to the next probable culprit.
Top of the list in chasing the P0087, is the filter head. You can try a rebuild, but not all potential leak areas are serviceable.
Absolute fix is a new filter head assembly; head, filter, sensor, heater, harness, etc.
Two bolts and drop in, easier than replacing just the filter alone.
Also include the two fuel lines to/fro it too. If the head has decayed (leaks), those two lines are tired too.
A restricted filter can and will expose other leak areas.
Benefit of the lift pump is positive pressure vs vacuum, which helps eliminate sucking air from various points and weak/worn lines sucking flat.
Which can be looked at as a band-aid. Find the leak problem 1st.
Rail plug; bottle test will show if the FPRV is leaking by. But again, if it leaks, it's not a consistent thing. Hotter fuel after driving a while can trigger leaks you won't find with cold fuel.
There's 'bout 1/2 a doz things that can contribute to the fuel limp; maybe it's just one thing, or more likely a combo of all of the above. A little leak here, a little leak there, and the CP3 can't keep up. And, it's rarely consistent in testing.
2007 LBZ 3500 LT3 DRW Crew Cab, Sulastics, Putnam XDR 15K, B&W 30K-Turnover GN+Companion, Ride-Rite Air...EFI'd by Rob
2008 LMM 3500WT DRW Crew Cab, 8 'Flat/GN, Workforce Alum Toolbox, Ride-Rite Air Use-ta Haves 2006-6.0 3500 WT DRW ExCab Flatbed, 2005-LLY 3500 LS DRW Crew Cab, 2002-LB7 3500 LS DRW Crew Cab I Live my life in such a way that when my feet hit the floor in the morning...Satan shudders & says "Oh shit, he's awake!"