I have been toiling with my '78 trying to get the heat to work right. The hot/cold door is shutting all the way but the small door that switches from heater to defrost doesn't completely seal because the foam is all dry rotted. Where do I get that chunk of foam or what did you guys do to fix the problem? Also, is the small door on the passenger side of the distribution box open when the defroster is on? Mine never opens at all and I used a vacuum pump to check the pod and it is okay, though the door is all shot to hell where it seals off. I've got it narrowed down to the heater core or the heater valve why I don't get burned out of the front seat when the heater is on. My '73 will flat burn the house down.

That "Chunk of Foam" can be found at about almost almost any hardware store. It will be sold in the weather stripping section or where they carry the garage door seals. It comes in a roll with a peal of adhesive backing (I used RTV to put mine on with).