Mine is a '91 300-SEL so we are not talking apples-to-apples.
Had a similar issue once. Small plate on top of mono-valve had loose screw. I could never see it until I put a pressure tester on the pressurized coolant tank.
You may need to do the same.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"