I agree that the sensor is useless if there's no pressure there for it to detect. I do appologise, I had misread some of what you had put in your first message.

All the spools are moving fully and freely on the valve pack?
There's not too much more I can think of without pressure testing the system. The only other thing I can possibly think of is that the system has sucked in air at some point, try removing the outlet hose from the tank and fill it up with as much oil as possible? It's a long shot but iv had it happen to me before on other linde models and is the only other thing I can think of without pressure guages.