so I have a 97 v10 3500 I got it a while back with bad head gaskets so I did all the work to replace them and all that good fun . Cranked it up and no oil pressure tried multiple different things (cleaned pick up ,ran a scope through the block,checked tolerance on oil pump,new pressure sensor,tried with manual gage,) and still nothing until today i ran a few seconds without filter and still nothing so I put a hose on the return where the filter screws to and blew into it and gained pressure so I screwed the filter back on and it had 50 psi for about a minute and then dropped pressure again so I tried again and same thing I can keep pressure for a minute or less then absolutely zero pressure. I also checked relief valve and it seems to operate properly

 

Is the spring on the pressure relief valve broken? Are the bolts that secure the oil pump plate nice and tight? Is the crankshaft key sheered for the oil pump?

 

the bolts are tight i re sealed the oil pump and the pump dont have a key it is oblong shaped and fits snugly on the crank
and spring is not broken

 

Is the pump slipping on the crank??

 

not that I can see there is no evidence or scoring on the crank or oil pump

 

Then it has to have an air leak somewhere between the pickup, and the pump.