Auto Air Conditioning & Heater Repair: A/C runs on vent setting, vacuum line, gmc 1500

Question
86 GMC 1500
When I move the lever on the a/c,vent,heat selector to vent the A/C continues to run. I have to go to off or heat or def to get the compressor to shut off

Any suggestion where to start?

Answer
Several vacuum and electrical tests need to be performed in order to determine which part is at fault. Could be:

faulty vacuum line(s).
faulty vacuum rotary valve in the dash.
faulty electrical switch in the dash.
faulty vacuum diaphram under the dash.
corroded electrical terminals.