HVAC stuck on Bi-Level

[skylark68]

My Suburban's hvac system has worked fine up until a few weeks ago. I never put the system into bi-level but for some reason regardless if the switch is in vent, heat, or A/C, the airflow comes out of both the upper vents and the heater vents. Is something stuck? It's starting to become a little bit of a problem with the temperatures rising. I have to put the fan on high just to get enough A/C to my face when I'm driving home from work.

[jaredts]

Do you hear any hissing when you slide the selector?  Did you make sure you have vacuum from the reservoir on the passenger side firewall and no issues there?

[skylark68]

I can hear a hissing sound when the selector is moved. I'm not sure about vacuum from the passenger side though.

[PromiseKeeper]

How far back did Chevy use the blower motor resistors in the ductwork near the heater fan under the hood? I've repalced them in 90's vehicles that had this problem. Usually the connector has a bad spot in it.