I'm not sure, but I think there are two paths from the pump, one up to the reservoir and the other goes to the main, so I think you would still have oil at the header, but just not to the reservoir if the valve was bad? but I'm not positive,
here's a thread,
No Oil from Lube Pump Need Help
search his user name to find his other posts, looks like his pictures in his sig are still good(I had to search back and find his thread)
