climate control,126

[larry perkins]

ok guys and gals i need an assist.
i did read some of the threads on climate control but didnt find my situation.
i have a 1990 350sdl and the problem is when the ac is running and the temp is set at 72 deg f,all is fine,air is flowing out the center vent,once the temp dial is lowered then the fan picks up to faster speed but after about 5-10 seconds all the cool air is directed to the defroster vents. i tapped into the vac line that supplies the switch over valve block with a vacuum gauge and sure enough i can see the vac reduce from 15-20 down to about 5 in and then the flaps move the air to the defroster vent.
drawings dont show any control valve from vac pump to switchover block.
any insight is apperiaced.
larry perkins lou ky