Brake fluid is often neglected on most vehicles. Accelerated wear/oxidation of brake system components is directly connected to lack of regular fluid change.

Brake fluid is hygroscopic (water loving)
some info: http://en.wikipedia.org/wiki/Brake_fluid

because of this, it is important to change it regularly. Water will destroy the hydraulic system.

Sadly not all mechanics or service providers will insist upon brake flush. Think about it. After the system fails, there is more money to be made replacing major components.

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85 300D 3 pedal. Current project.
83 300TD (need rear wiper assembly dead or alive)
84 300SD Daily driver
85 300TD almost 400k miles and driven daily.
98 E300D *sold
86 300SDL *sold and made flawless 10 hour journey to new home.