Auto Air Conditioning & Heater Repair: Air Not Blowing in the Middle Vent., gmc savanna, vacuum diagrams

Question
I have a 1999 GMC Savanna Conversion Van.

I am not getting air flow at the middle vent.
My air or header blows on the floor and the windshield.
My air condition or header is not blowing in the middle vent.

Answer
The first thing you want to check is the vacuum supply to the HVAC head.  The mode control works off vacuum diagrams and there is a small black vacuum tube the travels from teh engine to the passengers side evaporator box and then into vehicle.  They are prone to cracking or coming off the reservoir.

Wayne