HEATER PROBLEMS

[Old Dave]

THIS IS IN AN 86 C10 PICKUP. HEATER HEATS OK, AC COOL OK. WHEN THE HEATER IS ON AND IN HEAT POSITION 80% OF THE HOT AIR COMES OUT OF A/C VENTS IN THE DASH AND COOKS YOU FROM THE WAIST UP. VERY LITTLE AIR COMES OUT ON THE FLOOR. WHEN YOU GO TO DEFROSTER IT DOESN'T BLOW HARDLY ANY HOT AIR ONTO THE WINSHIELD BUT BLOWS OUT THE A/C VENTS. VERY HARD TO DEFROST THE WINSHIELD IN THE MORNING. THE GM DEALER SAID THERE IS A VACUUM SELCTOR VALVE ON THE HEATER/AC CONTROL PANEL. HAS ANYONE EVER HAD THIS PROBLEM AND HOW HARD IS IT TO REPLACE. I'VE HAD THE CLUSTER OUT BEFORE BUT NEVER MESSED WITH THE HEATER CONTROL PANEL. AUTO PARTS STORES DON'T CARRY THE VALVE..DEALER ITEM. THE ONE IN MY 86 SUBURBAN HAS STARTED TO DO THE SAME THING ONLY NOT QUITE AS BAD..YET.
I'VE CHECKED THE VACUUM ACTUATORS AND LEVERS AND THEY ALL SEEM TO BE WORKING AND NOT BINDING. IDEAS??