Anyone have a link to an internal view of a W123 brake booster? If not, how about a description of how it's made and how it works?

I pulled the booster from my 240D parts car and noticed some wetness inside. I was surprised to find a lot of dark reddish fluid coming out of the vaccuum port. Why is there fluid in the vacuum circuit?

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked