My AC when i start my truck will blow through my defrost vents and then after the truck "warms up" or has been running for a little bit it kicks back to my regular vents or whatever I have it on . I understand this to be a vacuum problem maybe? but what could the issue be ?

 

There is an electric vacuum pump mounted under the hood on the passenger side inner fender well. With the key on the pump runs until it reaches a set vacuum in the system then shuts off.
If the vacuum reservoir is empty or low it takes a while to reach the shut off point.


I would suspect there is a vacuum hose either off or broken and the pump has to run longer to reach the shut off point or develops a high enough level for vacuum operated controls to work . A thorough check for any leaks should be made and repaired and the system should work as designed.

 

Could be the pump itself, supply lines, ESOF if equipped can be another leak point..