Defroster Duct Doesn't Blow

[75sierraclassic]

Well its starting to get a little chilly in the mornings and I recently noticed by defrost vent isn't working. What do you guys think is the most likely culprit? The hot/cold lever works, the A/C still turns on, it just seems that some of the dampers aren't working that redirect the air to the defroster vents. Vacuum leak somewhere, bad diaphragm, vacuum switch? If the hot / cold lever still works does this mean that the vacuum switch is good and its a diaphragm or vacuum leak somewhere?