As you've changed out the oil pump. Either there is a blockage in the oil suction line or an air leak in it. Could also be a blockage down stream of the pump. Hows your oil pressure relief valve. Also verify that your oil pressure gauge is correct..
I vote for an air leak on the suction side. Generally speaking a positive displacement pump doesn't need priming unless the pump is badly worn. If the pump was a new install and pre oiled it should do the job.