I have an old NEW hOLLAND MODEL 390 tubgrinder that I had the hydraulic motor rebuilt. I used to run this tubgrinder with an old IH 806 diesel. Now it won't begin to turn the tub. My son had it over to his place in a neighboring state, and ran it with a much newer Case IH model, and the tub never offered to stop once. My little operation don't justify a high dollar tractor just to run a tubgrinder for 40-50 cows. Any suggestions on increasing pump pressures, etc. ??