I would check with the guy that rebuilt the Ranco . He may have some ideas. You have a beautiful heater and it should work as new. If it is not too much of a job for you I would run the hose from the water pump to the Ranco heater valve inlet. Then if you have an extra heater hose laying around run that hose from the heater valve outlet back to your water pump. Fill with coolant making sure you have air out of the system . This will bypass the heater itself and let you test the valve only . I would set the valve to the open or Warmer position . Start the engine and bring up to temp (make sure no air pockets) . If working correctly the inlet heater hose temp right next to the Ranco and the outlet hose right next to the Ranco should be nearly identical in temp . If you have a heat temp gun that would be the best . If not the old hand on the heater hose test should work. They should be nearly identical in temp. I think you said you tested the Ranco valve but to me it seems like you are not getting flow thru your valve as the evidence shows when you bypassed it. Good luck . Should be easy to get this solved.

ppp