Please help
 

Okay so I have a 1972 402 then I have rebuilt and I cannot get it to hold oil pressure I have replaced the pump the filter bearings and everything that you can replace it has to do with oil I put a high pressure oil pump on it new oil new filter yada yada and first time I put it together and had good a****** pressure not 60 70 PSI but it would warm up and fall to the zero not immediately fall to zero but gradually so I ripped it all down went through it all again put it back together in a still can't get it hold on pressure is there something I'm missing