3800GUY,

You are not alone in suffering from this predicament. Check out this previous conversation on the topic you asked about.

Boils down to your vacuum operated motor needs some TLC and quite possibly a rebuild.

Others will chime in here with more recent info and suggestions soon.

Dan


~ Dan
1951 Chevy 3 window 3100
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"My Grandpa Carl's Truck and How it Became Mine"
1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver)
US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)