A/C vent - 1995 Chev K1500 Z71 5.7 ltr 350 - 145,000 miles

Look under the hood and locate the black round vacuum canister. It has two vacuum hoses attached. One leads to the intake manifold and the other inside to control your vent selection. They are hard plastic and often break and cause problems check them out. Good luck

Thanks for the reply about checking for vacuum leaks. Done and found no leaks. Repaired a damaged connection. Question! What other role does the A/C button serve other than turning on compressor? Air flow is fine thru dash until this selector is pushed, then no air. The vent control door on the floor works fine in all settings unless the A/c button is pushed. When in dash position and A/c button is pushed "in"- the floor vent door will not completely close. Trying to narrow this down to see if maybe it is sometime of electrical relay? What do you think?

The ac button provides a signal to the ECM for engime idle up with A/C on and turns on the compressor via the ac circuit.

All doors are driven by a small motor on the side of each pivot. It is a small black flat component with a connector. It can be removed with 2 or 3, 7mm screws. I suspect one is binding or the flap is binding, They are about $40.00 each. If it was me, I would change one from another area as they are all the same. This will isolate if it is the flap/door or the motor. Good luck

Flap and control door works fine in all other setting, moving to completlely close or partially close. Could what you are telling me still be the problem? I'll try everything you say. I think I understand that it could be binding just in that one position (dash vent on and A/C on). Thanks!