Auto Air Conditioning & Heater Repair: a/c vent problems, floor vents, steep grade

Question
I have a 1990 cadillac sedan de ville, v8.  My a/c and heater work fine, but the front vents do not blow air, only the floor and the defrost vents blow air.  The a/c control seems to be working fine, but there is no vaccum to the vaccum valves to open the front vents, when I put the control on vent.  Any suggestions on what could be the problem?

Answer
How about when you go up a steep grade and or under heavy acceleration and when you have the panel set to FLOOR VENTS does all the air divert up to the DEFROSTER VENTS ?.

Has there been any work of any kind what-so-ever done anywhere on the vehicle prior to this problem ?.

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