Heater rebuild

[53Rick]

I have a '72 GMC 1/2 ton with a 402 engine and factory heating/ac. It has heater issues and I'm looking for some advice as to what needs to be done without spending a fortune on parts that likely don't need to be replaced. The control cables seem to function okay. The heater blows hot air, but little to the defrost (more to the driver's side than the passenger side). Regardless of which position the levers are in, I don't feel any change in the amount of air blowing to the defrosters or through the dash vent above the control assembly. Most of the air comes out through that dash vent. It appears that the various gates/doors do not function properly to redistribute air. I would think there would be a manual for these heaters aside from what is in the shop manual, but haven't found one. Before I start ordering and replacing vacuum switches, relays etc I'd like to know what most likely is the problem. Any ideas would be appreciated. Thanks.

[Mister-B]

You probably have one or more heater hoses that are ripped, or have disconnected from either the defroster vents or the heater box. Just get on the floor of the cab and shine a light up. You'll be able to tell pretty quickly
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[53Rick]

Nope. They are all connected. That was the first thing I checked.

[LEEVON]

Sounds like a broken flapper door (within the diverter box) or disconnected cable. The factory assembly manual has excellent diagrams. I couldn't have gotten through a rebuild of my a/c unit without it. My flapper door was good, but the cable was seized up. If you can re-use your cables then go for it, they have conformed to the right twists and bends over the years. Hard telling until you get everything disassembled, but there is alot that can be cleaned and repaired starting with the control head unit, vacuum lines, cables, ducts, and flappers.