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.