1986 Scottsdale K20 8600 GVWR 4x4 5.7L

Defroster not working correctly. I get about 10% of the air flow to it. I've checked the hose routings behind dash. It is all vacuum lines going
off the controller. I get lots of air flow through the vents on the dash only. All the vacuum lines go to a unit that I believe has a diafram in it. Trying to find out how to fix this. Thanks
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