Well, I know you asked for the "big brains" and that's certainly not me, but in the meantime, here's my thoughts, having just finished an engine swap and HPO system work.
If the HPO system has been drained, it will take a LOT of cranking to prime it. I cranked mine long enough to drain my brand new batteries before it even tried to fire.
I would first pull the FICM relay out, since cranking is quite rough on the ficm. Sounds like you have a way to monitor ICP and IPR, so keep an eye on them while you're cranking. With an empty system IPR should be ~85%, meaning the IPR valve is allowing full pressure through. ICP will be low, probably zero until you get most of the air purged out. Once you see ICP above 300-500, go ahead and put the ficm relay back in and try to start it again. The truck is programmed to not fire the injectors until it sees ICP above a certain value, around 500psi, I think.
As for the ICP reading with the sensor unplugged, If I'm not mistaken, when the computer doesn't see a sensor input, it uses an inferred value within the acceptable starting range, thus your 953psi.
Hopefully this helps, and maybe someone more knowledgeable can weigh in and help you out (and correct me if I'm wrong).
If the HPO system has been drained, it will take a LOT of cranking to prime it. I cranked mine long enough to drain my brand new batteries before it even tried to fire.
I would first pull the FICM relay out, since cranking is quite rough on the ficm. Sounds like you have a way to monitor ICP and IPR, so keep an eye on them while you're cranking. With an empty system IPR should be ~85%, meaning the IPR valve is allowing full pressure through. ICP will be low, probably zero until you get most of the air purged out. Once you see ICP above 300-500, go ahead and put the ficm relay back in and try to start it again. The truck is programmed to not fire the injectors until it sees ICP above a certain value, around 500psi, I think.
As for the ICP reading with the sensor unplugged, If I'm not mistaken, when the computer doesn't see a sensor input, it uses an inferred value within the acceptable starting range, thus your 953psi.
Hopefully this helps, and maybe someone more knowledgeable can weigh in and help you out (and correct me if I'm wrong).