I didn't see this question asked after a search so I thought why not. I have a 97 GMC Sonoma with 90k miles that I barely drive. Since the hot summer is coming fast I went to use the AC and I am only blowing warm air from the defrost and floor vents. After searching the site I did run across a question regarding the Sonoma's brother, the s10, that mentioned to test the vacuum lines. I was curious to know if checking vacuum lines can be done by spraying a soapy solution with a spray bottle while the truck was running do the trick?
As stated in the comments, the doors were working intermittently at times when it would blow through the dash and cycle to only the defrost and floor. First step from past experience I did examine the vacuum lines such as next the canister. I did notice they appeared to by dry rotten so I went to O'reily's and purchased new vacuum lines for the location for the top right driver side where a T is located. After replacing some of the vacuum lines the issue proceeded to be less then a month later completely went out.
My interest is trying to figure out where to test the vacuum lines properly. Sometimes, as it could be a related issue, there is a bubbling noise coming from the passenger side dash which I believe may be the heater core but I am unable to determine if the heater core goes through the vacuum system.
Pic of location:
Per request, the controls are electrical. Here is a pic of the controls. I do plan this weekend to further test the system:
Do note that no matter what position the AC controls are in they will blow out through the defrost and floor vents.