I wanted to post a quick update on the solution to the squeeky brake pedal in my e420. The replacement of the booster ended up resolving the issue.
(found a used one for $100 shipped)

The source of the problem (just a guess) was a blown gasket inside the booster causing the rubber components to swell following their exposure to excessive brake fluid from the master cylinder, in turn causing the most annoying squeeking sound in the world.

Removal and install of the booster was about as snug as it gets but it is doable with the removal of the master cylinder and loosening up of the fuse box. Total time for the job was approx. 2 hours.

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1995 E420
1998 SL500 Sport
2009 E350 Sport
1995 S500 Coupe
1992 500 SEL