Haven't heard of the weak spring issue. Can you elaborate? I installed one or two MTC inserts (~$30 ebay) years ago and the rubber diaphragms are still intact, which seems to be the main failure in these. Usually the problem is "full heat". Sounds like pressure can overwhelm an opening spring to keep the valve shut. I removed the aux pump in both my 300D's, so perhaps less dP in my cars. Still more than enough heat for our climate (orange trees).

A few basics, have you felt the heater hoses? Perhaps the heater is doing its job, but is overwhelmed by outside air coming in the climate door (worse at hwy speed). Try blocking off the cowl vents. Is the AC compressor engaging too often?

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans