I'm curious about this phenomenon since the SD acts in a similar way without the use of the aux. pump. The replacement pump was affecting the monovalve in a bad way and I disconnected it a couple of years ago. The results are such that the defrost air is the coldest of the group. For some reason, unknown to me, external air is getting to the defrost outlet and is not getting to the other outlets in the vehicle. When fully warm, the defrost outlets remain colder than the others but not full cold. There is still sufficient heat to keep the windshield defogged.

I was under the assumption that the lack of sufficient flow from the pump was the issue on the SD............but, maybe it's not.