The heater control valve will not close. I replaced it with a new valve and, it too, will not close. I checked both valves, visually, for full range of movement (when they were on the bench) and they both smoothly opened and closed. With the new valve installed, heated air discharges with the heat control lever in the cold position, just like it did with the original. Installing an inline on-off valve, as suggested by some, does not cure the "regulation" problem i.e. cold-warm-hot, it only provides a cold-hot choice. Is there a way of adjusting/ calibrating the valve and/or the control lever?