I just went through pre-oiling my 5.9 Magnum motor. I installed a new high volume oil pump, Milodon pickup screen and dry oil filter. I spun the pump clockwise with my 1/2" drill and quickly produced 75 PSI of oil pressure on a mechanical guage I connected to the motor. Did you have a gasket installed between the pump and the rear bearing cap? You stated that you noticed bubbles in the oil when you pulled the pre-oil tool out of the pump. Sounds like your pump is pulling air on the suction side of the pump somewhere. Most likely place is at the rear main cap. Also the pickup tube may have a crack in it. Just some things to check..