If the valve is bad, an o-ring won't fix it. We do spring conversions on P38 Rangies all the time- $795 for the parts and an afternoon of work- and without fail the customers always say they like the ride with the coils better.
The blend doors are a PITA to work on- if it needs a module that's easier, lots of junked P38's to get used parts from. Usually it's the blend doors. On one, we just parked the doors so the air went to feet and windshield and they were happy with it fixed on that setting...
Either that or sell me the truck cheap and I'll fix it and keep it.
