GM-GMC: A/C controls not allowing air from dash vents, vacuum leaks, vacuum leak

Question
have a 1996 Blazer which the a/c control will change the airflow to all selections except vent. How should I test to find what is causing the problem?

Answer
With the engine running check under the hood for vacuum leaks.  My guess is you have a vacuum leak in one of the hoses which supplies the control head.  If you can't find a leak, remove the control head and start the truck to see if you have vacuum at the controller.
Try that and we can go from there.
Hope this helps