The mono valve has a replaceable insert. Many sites/dealers may list it at ~$40. However, I've seen it priced today at ~$15. The entire valve assembly is probably near $200, if it is even still available.

Your last post gives me some hesitance. It almost appears that, with the drop in temp, that coolant is entering the heater. Are you getting air from the knee vents? Where are you checking for heat?

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Sam

84 300SD 350K+ miles ( Blue Belle )