How are you determining that you have no oil pressure, By the gauge on the vehicle? Is the engine noisy? Losing prime is not very common, and the fact that it is a used engine, you don't know the history. Pull the oil pressure sender out of the front and start it to see if it truly does not flow. There is no procedure for priming, there was a pre-luber issued to dealers in around 2002, that was specifically called for when installing a new, remanufactured, or serviced(anytime crank or conn-rods were removed) engine.