Regarding the moral support, I was thinking your test of the valve before you installed it was a good move, and that your test was true. Maybe not. Coolant at 200F might perform differently than your test method.

So, I was hoping you'd clamp the hose and find no difference, which would lay the blame on perception, rather than a faulty part. It has been awfully hot in the recent past. The circulated air has opportunity to be heated without the aid of a heater core with engine coolant flowing through it.

Take it easy and one step at a time. With all those other things you have to do, you might wind up checking it again in the cool weather, and coming to a different conclusion. I can tell you from experience, having a fleet to maintain can get in the way of science experiments diving deep into diagnosis.
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Art Benstein near Baltimore

You can easily judge the character of a man by how he treats those who can do nothing for him. ~Malcolm S. Forbes