Defrost, Vent, Heater Air Flow Chevy Truck

I only get air through my dash vents. Defrost, floor air does not happen. The controls are all vacuum line control. Lines go off back of control module in dash to a single diaphram valve. I've checked all the
hose/plenum connections and they are fine. 1986 Chevy Scottsdale 3/4 ton 4x4. Any ideas out there please?
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IM not by any means an expert,but look at your vacuum hoses to see if theyre in good shape,not all cracked and detiorierated. maybe?
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