Pump clearances have to be tight enough to create sufficient vacuum to pull cold oil up to the rotors. Also any minor leak in the intake side of the pump obviously is bad. So IMHO get a priming rod and test it. If it can't pull a prime fairly quickly at 600 drill rpm, I would be concerned. Scored pump rotors and big clearances have caused me problems with a single line system. Also sitting for a long time allows oil to drain off the rotors, making it tougher for the pump to pull a prime.