Losing Prime, could it be the schrader valve?
 

So I have a 1990 F350 7.3, auto. This is my normal driver, but due to my job it can sometimes sit a while at a time.

Prior to this week it sat at the airport for almost 3 weeks, when I left the temperatures had gotten down to -50F. Since then they were subzero until this past week and now we're back into the positive temperatures. When I got home it was 10-15 degrees and I had trouble starting the truck (usually it's fine until around zero). It did eventually crank, and I chalked it up to being cold soaked and sitting so long.

The difference came the next day, it did get below zero (barely), but when I woke up and saw that I plugged the truck in and it was plugged in an hour or so and the engine was nice and warm when I tried to start it. Once again it took a lot of cranking until it finally cleared itself and ran. I let it run for a while and then shut it down and parked it intentionally until this morning. Once again it seems to have lost prime and doesn't want to start.

Truck has (relatively) new orings on the return cups, and new return hoses. The injection pump is an arctic pump approaching 2 years old and the glow plugs and controllers are new. No signs of any leaks. Lift pump is also pretty new and fuel is appropriate for the arctic temperatures as well as treated.

My question is if it's unheard of for the schrader valve on the fuel filter housing to allow air in but not leak fuel. I don't have any fuel leaks or sign of wetness anywhere. With the truck running I can put some pressure on the valve and get a nice flow from the lift pump. The other question if anyone has a part number for a diesel rated valve so I can replace it and try that. The truck isn't loosing prime too badly since it's able to reprime itself without being bled, but I need it to start faster than it is. I'm a volunteer firefighter as well and I prefer to respond with this truck, but it's not happening if I can't count on it to start.

Any other suggestions are welcome as well. I know the cold temps mess with a lot of things, I've had to tighten a number of hose clamps to stop coolant leaks just from the temperature swings. It seems that a few hours aren't enough for it to lose prime, but sitting overnight is.

hope u got figured out before now if not me and my son just fixed my truck today with the same problem we ended up bypassing the water seperater was the problem sucking air
hope that helps you

Was this a 6.9? On my 7.3s the water seperator is part of the fuel filter, so I can't really bypass it. I might go to a one piece setup versus the current 2 piece though.

I still haven't gotten the problem fixed, but there has been some stuff that came up that was more important so the truck got on a back burner. I might also have a glow plug problem now after the last time I was trying to bleed the system (they aren't staying on very long). Glow plugs and controllers are pretty darn new too...

is yours turbo?
if so the injectors operate off the high pressure oil pump,but
how old are the batterys?
if it dont turnover fast enough it will be hard to start to