Why does my 7.3 idi lose 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 has front and rear fuel tanks and both have plenty of fuel, and it loses prime under either one.
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.

 

[Sidewinded_idi]

I would say bad lift pump, you say it's new but could still be defective

 

[bj_idi]

have you checked or changed fuel olives? you could be pulling air into the fuel system

 

[Guffey6]

Do a clear hose test to see where your air is coming from. Does any of your new return lines look wet? And consider doing the facet electric fuel pump mod.

 

[Martian1743]

What is a "facet electric fuel pump mod"??

 

[Guffey6]

It's an electric fuel pump. The brand is facet 40222 I believe. Anyways, it will replace your mechanical lift pump.