I have checked other thread on climate control but I havent seen this problem discussed. I have no heat in my '84 300 TD until I have been driving at freeway speeds for 20-25 minutes. When I located the Mono-valve I noticed that the 1.5 inch rubber hosebeside it is broken just as it come through the fire wall. I taped it up thinking it might be related to the problem but nothing changed. Any suggestions.