The center vents in my '81 Euro 500 SEL (W126.037) stopped blowing cold air and after troubleshooting it with the help of the diagram posted earlier in this thread, I found an easy fix. An original vacuum hose rubber Y connector leaked. It was made by WOCO, MB part 1170780145, Mercedesshop has the replacement in stock:

Item Number: D3011-73182
Main Category: MERCEDES BENZ
Sub Category: Vacuum Hose Connector
Description: Vacuum Hose Connector
Manufacturer: AAA:LEM
List Price: $5.25
Your Price: $ 3.26
Core Price:
Weight: 0.010

Quantity Available: 9

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W163.154, M112.942, 722.662, ZF.970.402