it seems to me that many links in the chain on your climate system are old and weak. there may have only be one link that was broken but when you fixed it the next weak link gave way. my experience is on a 380sl climate system so this is what I did:
with the glove box out and the instrument panel out i replaced all the vacuum tubing that controls flaps, also replaced all the check valves. tubing is cheap, even if you get the diff colored ones for each vent. so far total cost was around $18.
next came the actuator pods. all of them tested good with my-t-vac and held good vacuum for a few minutes.....except the one controlling the center vent. that one couldn't hold a jello vacuum, and that one is inside the heater box (means taking apart the whole dash to get at. since I can reach the center flaps with a wire hanger, i pushed the flap half way open and can live with a mild amount of air coming thru. by the way, that pod was held in place by two rivets, but the plastic housing around that area cracked so even if the pod held vacuum it is still just floating around loose.
I sent the push button unit to proforma in florida to have it rebuilt (resoldered) and now that works fine. cost = $135
same can be done with the blower control unit at similar cost. there is nothing else inside these units to break except solder joints.

still have hot air always? replace the monovalve. cost is $30 and takes 5 minutes to do.

by this time you will be very intimate and familiar with your firewall. tight places that made your hands ache with nicks and scratches will now feel roomy. and oh, yes...from time to time you will drop small metal objects and have side jobs of trying to find them, but thats another story.