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If the return loop is left open, the heater core will be exposed to the system pressure, somewhere between 12 and 16 psi depending upon the radiator cap.

Yea, true I was not thinking as much about the pressure as much as flow. Not to mention you would be losing the pressure in the system from the leak, which is not good for cooling system performance. 

Dennis you may try loosening the packing nut on the valve that will not move. Sometimes if you loosen that it will allow you to turn the valve. Just make sure you tighten it back or it will leak. The packing nut is the nut that is on the stem of the valve.