What year is your truck?The early 99's had issues with the HPOP.The easiest way to check this to screw a gauge into the oil gallery rail on the head.This is the rail your ICP is screwed into.Like was said before you need at least 500psi to crank.The thread size on the rail is -5 JIC male.I have some fittings and a gauge if needed.I will PM you my number.I ran into one truck acting like yours is and it turned out to be the HPOP.
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1964 Impala 468BBC,T350 transbrake,4.30 9",10 point and 10.5 M/T's
1996 F250 4X4 12V Cummins,compound turbo 63mm over 75mm,cam,520cc pump,full billet 4R100 563hp&936ft.lbs w/a bad fuel pump and on a dynojet...just an old hunting truck 
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