A/C guys....I need your help please.

[tucsonjwt]

I don't know what is wrong with your truck, but here is what I experienced with my 83 C20.
Airflow to the upper vents in the dash was restricted by the hoses and vent ducts not being completely connected. I took out the dash pad, glove box, etc. and started following the ducts and hoses to make them secure. I used HVAC type silver tape to wrap up the connections to make sure that they did not slip loose. I also added some foam type house door insulation where the ducts did not seat firmly against the dash. The factory used foam to help seal these gaps also. If you look at the duct system, you will see that the ducts and hoses just barely fit together loosely - not a very good set up.
I have my carburetor set to a low idle. The fast idle solenoid on the carburetor for A/C operation does not add much to engine R.P.M. Still, when I come to a stop at a traffic light, the engine revs higher than normal, when the A/C is not on. I think the throttle linkage is sticking somewhere, but I just jab the accelerator a little and it returns to its normal low idle. If the A/C is on, I just don't jab the accelerator and the idle is high enough to keep it from stalling. As far as I am concerned, as long as the engine doesn't stall at idle with the A/C on, I don't care if the fast idle solenoid works. I would look at capping off that fast idle solenoid and increasing the throttle screw slightly - or replace the fast idle solenoid.
If your compressor will not shut off when the dash control is in the off position, I would think that the switch is not functioning in the off position.
Obviously, an auto A/C guy is the person to talk to, but they usually don't work cheap, and you can replace a lot of parts for the cost of a diagnosis/misdiagnosis from a professional. They also make a lot of money off their markup on parts - if they can find them. I would check your ducts/connections, fiddle with the fast idle solenoid, and price a new HVAC switch or possibly replace the whole assembly, before I spent money on a pro.
BTW, the vacuum lines under the dash are a tiny diameter and I have not found anyplace that sells them. Also, it looks like major disassembly would be required to trace them all.
Good luck.