Went through this recently. Post 99
Got a 1985 300D turbo
Sounds like you need to check your center vent vacuum pod. The easiest way is to mity vac test it right at the solenoid. The hot air blast when slowing down is likely a torn monovalve diaphragm. Easy to disassemble and inspect.
Zip ties work great. Check out my w126 center vent repair here. Not quite the same as the 300d but same general idea.
Fixed AC center vent flap vacuum pod 300sd w126