1986 Dodge pick up. when selecting ac vent heat or defrost. Air will come out of main vents but not out of heat vent or defrost vent

I would say that vacuum supply line has come off or cracked somewhere. there should be a skinny little plastic tube that goes in through firewall. follow this backwards to the vac. tank. if all ok, follow vac. tank supply hose back to engine. if all ok, then problem will be inside vech. poss hose unhooked at control or tube pinched. hope tis helps........

Did you ever get this figured out? I'm having tbe same issue on my 85.