Have a 71 Elco, with a BB,AC Auto Tran. The air control, is not working right. When it is on vent I get bilevel, and when I go to AC it works right. Been this way since I bought it. Now the defrosters, are doing the same, about half defrost, and half heater. I have checked the vac lines on the swith manifold, and changed it to see if any difference, still the same. Any Ideas?

Open the hood and look in the cowl box on passenger side, See a vacuam line in there? Check too see if it is broken at the firewall. Mine was an repaired it with some tubing, Heat Defrost work great now. Jeff