03 Peterbilt 379L with dust problem in cab

There is a hose that lets the condensation run out, check to make sure it has a loop in it.

 

Never, Ever leave the fresh air vent open! I don't care what filter you use they let a ton of dust through! The day I buy a truck I take it home and seal the fresh air vent closed.

If I need fresh air I crack a window down.

 

Had the same problem. It was the actuator motor that open and closes the fresh air door behind the air cleaner. It was open all the time. Check to make sure door is closed. I leave mine in recirculate all the time. No more problems

 

I had the same problem too but, not so much with dust, I was having problems keeping my windows defrosted. Turns out the rivets that hold fresh air door on the shaft corroded and the door actually fell off the shaft. Had to remove the air cleaner and pull everything apart to get the shaft and door out. I then spot welded the door to the shaft to hopefully keep this from happening again. Defroster works great again.