95 Blazer vacuum

My a/c, heat and ventilation all work normally except when my vacuum drops. When the vacuum is low, I just get defroster action. I live in the mountains of Colorado and do a lot of uphill driving which keeps the vacuum low. How can I increase my engine vacuum or keep the air system from going to the default defrost position?

I have the same problem. I saw another forum that said it was a problem with the vacuum lines behind the dash where they connect to the controller. I would be interested to get more input on this subject.