So been fiddling with the vac lines under the hood. When my blazer is cold (Ontario Canada, so winter) the dash vent blows strong. But once it all warms up, it fades to the four vents facing the seats, no matter where i move the control. Replaced the lines from the canister in the fender to the check valve. I'm stumped. I have lots of heat just can't control where it goes! If i move the vent control while its still cold the air flow will change vents , but once it warms up it just moves back to the 4 dash vents on its own.It slike its losing some vacuum once its warmed up. BTW, the 4WD still seems OK. switch it over from 2WD and light is steady not flashing, so i assume it still has enough vacuum to operate properly.

 

I'd check all of the vacuum lines under the hood. They can dry-rot and leak even if looking ok.

I have a link to a thread I created in my signature I'll turn into a sticky-thread someday on the vacuum hoses for the Blazers with 4-button transfer case. Another member added info on the three button hoses.

But back to your symptoms. I have the same but only when doing a long pull up a hill when the engine vacuum is reduced for an extended period of time. Like you said it is as if the vacuum is "used up." And this is after replacing all the underhood vacuum lines, the connections at the intake plenum that supply the vacuum, the PVC valve, the solenoid actuated vacuum distrubution control. (I have the "Computer Climate Control."

I checked all my lines in the engine compartment with a vacuum pump. I even checked the hose that goes to the HVAC system with the engine off. Everything held vacuum at 15", with only the connection to the vacuum reservoir dropping about an 1/8" over 10 seconds.

The next step would be to check all the under dash connections and actuators. I'll be doing that in the spring.