Here is a quote from SRM.
"We frequently get asked the question about wet sumping.
We DO NOT recommend fitting any such non return valves on the feed side of the pump as gear pumps are not designed to suck with no oil in the body. This can result in instant engine failure."
Actually, MOST gear pumps like MOST positive displacement pumps are self priming, even on a dry lift system that's properly designed with no air leaks on the suction side. However, they must be in good condition with close internal clearances between the gears and the side plates. In the case of a stock configuration Norton, the suction side of the pump is always flooded... therefore no issues. But as we all know from the horror stories, the problem arises when installing a spring-loaded self closing valve between the oil tank and pump suction that won't easily open (possibly due to contaminated oil around the ball/piston, or a return spring with too much force, or a combination of the two). In my opinion, you take your chances with these self closing valves. Even a normally closed pilot-operated valve, opening with pressure from the pump, would be risky because if oil pressure in the system drops for any reason....valve closes and motor is toast. :shock:
Upon further thought, I shouldn't even have mentioned a pilot-operated self closing valve. I don't think it would work here; even with oil in the line from the tank to the pump it would probably cycle open and closed until it finally stayed closed. So don't even think about one of those in the line...
http://www.hawsepipe.net/chiefhelp/pump ... _pumps.htm