80% Defrost only from blower

I have a 1981 300SD. Fixed my A/C. Now all my A/C blows out the defroster. I noticed all modes are 80% out the defrost ducts. Where do I go next? Vacuum? Electrical? Any diagrams available for either?
I do show 15" vacuum at the source. Low I know... seems to be the shift diaphragm on the pump. It also takes about 10 seconds to shut down after key off.
What's next?
thanks,
Bob

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