Hot on left, etc. is pretty common with all valves not just thermostatic mixing valves. The range of temperatures we need in the darkroom is roughly 65-100F 20-38C if you inlet water on the cold side is above 68 degrees as you dial down the temp you will be getting lower and lower flow, ditto for the hot side when you approach 100 if you water is only about 100 to start. That's normal. If you are saying that when you "crank" the temperature knob all the way to the left or right and the flow stops, that is also normal behavior. If on the other hand, you are getting these issues in the mid-range your valve may need a rebuild. I don't think Arkay (and Pako and Calumet, etc.) made their own mixing valves the actual valve under the label is likely a Powers, Moen, Watts, or other.