Don't know how true this is, but.... I have been told it's common to have the master cylinder go bad after bleeding the system via the pedal method. The reason is that the master cylinder will get water at the end of the bore, which will cause pitting. (My Jaguar had this pitting when I tore down the master cylinder.) In normal use, the piston and it's seal will never travel through the last say 20% (just throwing out a number here). When you bottom the pedal during bleeding, the piston seal can get torn up crossing over these pits. (Or when checking the pedal travel during and after vacum bleeding, prior to it's being firm enough to limit full travel) The cure is said to be throwing something behind the pedal, say a small chunk of 2X4.

Mack, this is true. A better way is to use a pressure bleeder (like Speedi-Bleed, from Falcotools.com) and not touch the brake pedal at all. I loooooove my Speedi-Bleed!

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Dave
Boise, ID
Check out my website photos, documents, and movies!

dave see my post on brake bleeder
Brake bleeder recommendation

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Current Stable:
1994 S500 v140, 210k miles, white with grey.

Former Mercedes in the Stable:
1983 300CD Turbo diesel 515k mi sold (rumor has it, that it has 750k miles on it now)
1984 300CD Turbo Diesel 150 k mi sold
1982 300D Turbo Diesel 225 sold
1987 300D Turbo Diesel 255k mi sold
1988 300 CE AMG Hammer 15k mi sold
1986 "300E" Amg Hammer 88k mi sold (it was really a 200, not even an E (124.020)
1992 500E 156k mi sold
etc.