7.3 idi loses prime

[Martian1743]

1991 F350 7.3 idi : It has a new lift pump, new return line kit, new Motorcraft Glow Plugs, and all injectors have been externally cleaned, and all seem to work.
It was running smoothly UNTIL I hooked it to a large RV. After driving 15 miles with no problems whatsoever, I was faced with a semi-steep grade. The truck made it 1/2 way up the hill and immediately died. It had lost its prime. I primed it 3 more times on the way to the campground, and dropped the RV. Then headed home and had to prime it 3 more times just to get it home. Prior to this scenario, it was running perfectly for a full month.
How could it lose it's prime over and over?
I thought possibly due to the load, then I dropped the load and still it kept losing prime.
Ideas? Experience with this issue? Thanks for your assistance.

 

[greenskeeper]

dual tanks? is one of them on "empty" ?

 

[Martian1743]

Yes, dual tanks and both have plenty of fuel. It loses prime in either tank.
Also, I have added a clear fuel filter prior to the main filter on top of the engine. That filter stays full of fuel, even when it loses prime.

 

[Thewespaul]

I added a check valve before the lift pump on most my trucks, it fixes the issue and holds a few psi after being shut off, makes for a real quick start

 

[madpogue]

I added a check valve before the lift pump on most my trucks, it fixes the issue and holds a few psi after being shut off, makes for a real quick start

Can you post a pic of it installed, and/or a link to the valve you're using?

BTW, can a mod move this to the IDI section?

 

[Thewespaul]

https://www.amazon.com/dp/B01FSSH7Y0/ref=cm_sw_r_cp_api_ZXNEzbJFGSVWM
This is pretty much what I got, my truck is at the yard but I can get a picture if needed, it's just inline right before the electric fuel pump. If you are still using a mech pump you can put it in the short rubber hose by the frame rail