98 blazer 4wd 4x4

I have heat working very well when the vehicle is first turned on and after about 5-10 secondsit sounds like a door closes and stops the heat from coming through. In defrost it will continue with heat but it has to be on high although it blows with little pressure. In the front vents it just completely goes out and blows cool air but the pressure does not change like the defrost. The feet air is the same as the defrost.I have had every possible flush, thermostat changed, and vaccum is alright under the hood. I live in ohio and its pretty chilly without heat. I am trying my best to stay away froma shop soall assistance isgreatly appreciated.Can someone PLEASE HELP ME!!!!

 

I meant 98 blazer 4dr 4x4

 

I have same problem on a 97

 

For both a 97 and a 98, the air temp door is an electronic actuator.

Here are the vacuum logic and a diagram of the system for a 98:


The system schematics are the same between the 98 and the 97:


And the problems being described by the OP seem to be a bit different than yours Anthony as your truck takes ~10 minutes to have a diminished HVAC airflow output while the OPs takes 5-10 seconds.