I think you may have a leak on the suction side of the pump that you cured when you switched pumps. I'd use it and see if the problem is fixed.
I just bought a JD 600D backhoe for a song that was supposed to be a runner with a bad starter. It had been sitting for 5 years I was told. No Go after curing the starter/slow crank issues, fresh fuel, block heat, etc. Now I find out that its been sitting for 7 or 8 years and the PO only had it running once and not well at that. It does have a fresh head and rebuilt IP so I am going to get it dragged home and jerk the head and pan and do a "rings in the driveway" job to it since I now believe that it had the head done after the HG blew, The bad part is that w/o the engine running no steering or brakes so it will be fun to load and unload.