Vaccum Problems?

I have a 1993 Chevy S10 Blazer 4WD 4.3L Vortec Motor. I've got some interesting seemingly vaccum-related problems with it. It started about a month ago when I was driving on the interstate and my air-conditioning suddenly switched form blowing out the main vents to the top (defog) vents. Shutting the car off and back on fixed it. A week or so later, my add-on cruise control started taking a few tries to catch, now it won't catch at all. When driving above 50mph, I can only hold the pedal down for about 10 seconds at a time. If I hold it down for longer than that, the air switches vents, and occasionally, the transmission will start to try and down-shift. I cannot seem to find the vaccum leak. Any ideas?
-chall

Have you found your culprit?

I have a 1995 Blazer with same motor and same problem. A/C comes out the defroster. Haven't noticed the Cruise problem, I'll try it today. Do you also notice a funny smell when defrost is on? (call me crazy but it smells like butter pecan icecream).

I have a 1995 Blazer with same motor and same problem. A/C comes out the defroster. Haven't noticed the Cruise problem, I'll try it today. Do you also notice a funny smell when defrost is on? (call me crazy but it smells like butter pecan icecream).

I don't know if you found your problem yet! I also have a 95 blazer and my A/C was coming out of my dash. I took it to chevrolet and they replaced the vaccum ball and a check valve (the check valve is the one straight down from the ball, it has 3 lines going to it.) I can tell you that if you take the vaccum line going into the check vave off you should get a vaccum release sound. The check valve was only $10.00