The seal between the master cylinder and the booster is a vacuum seal....there should be no brake fluid there. You have a master cylinder leaking internally. I would just replace the master cylinder. I dont know about rebuilding it.....master cylinders are only about $60.

good luck

Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)