Ford 6600 injector pump

I have a problem with this tractor starting up after it sits for 24-36 hour. Earlier than that it starts right up. Had a bad injector pump to start with. Replaced it with a rebuilt pump (Simms inline) changed both fuel filters and after priming the system it starts right up. But after I let it sit for a day and half or so you have to crank the engine for a long time and then it starts up. I have no leaks anywhere. So what could be causing this problem?

Haven't we already been down this road?
Previous post

Yes, we have but still haven't found a solution. I feel it is a problem with the rebuilt pump but they tell me no. Something is causing it to loose its prime.

You never replied to my most recent suggestions, which is why I don't care to offer up new ideas.

A 6600, assuming it has only the top tank, is essentially gravity fed. If the tank is full, the lines are not obstructed, and the filter is clean... it basically can't lose prime. It might lose prime at very low levels in the tank. Even with a bad lift pump it should still run.
What I'd do... check that you have strong flow from the tank at the strainer. Follow the line along to the lift pump and make sure it's clear. Could also have wear marks and pinholes in that line. Make sure you get good flow from the lift pump. Follow it through the filter and make sure you get good flow from the filter to the back of the injection pump. Beyond that... like Bern said, check the rack. It's probably sticking.
I know the problem sounds like a prime issue. If the tractor had a saddle tank where the lift pump actually had to LIFT the fuel, I'd definitely look very very hard at that. I had one do that... it had a tiny, TINY pinhole where the line rubbed the block behind the pump... and it would lose prime overnight. Eventually it just wouldn't lift fuel anymore. But you have a gravity feed as I understand it... so look at the rack. From what I've seen, Simms with bad lift pumps will still run on gravity at low load, but stumble once they start loading heavier.

Rod

Thanks for the info. If the fuel line from the lift pump to the filters wouldn't I see some wetness if the line had a pin hole in it? This is a new rebuilt pump so I don't think there is a problem with rack sticking.