Check the HPOP Resevior. I bet it is low. Because you have to bleed out the system. Which means you need oil. Sure when you got it back together you filled it up. But once you ran it that oil was put through the HPOP system now your HPOP is empty. Needs to be filled back up.
You can use an OBDII scanner but it won't give you an injector pressure reading. Especially with the truck not running.
If you are going by the oil pressure gauge on the dash, that is engine oil pressure not the HPOP pressure.
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