SV,

I have not heard anyone explain why the shuddering and so on you describe will go away if you address the master cylinder.

Is the feeling coming from the front brakes or the rear? Do you have to pump the pedal to get the car to stop? Does the pedal sink slowly to the floor? How do you know the discs are not warped? When was the last time you changed pads?

I would be concerned as the shuddering is originating at the brake on either both front wheels, or at least one of them. A vacuum leak would explain needing to push really hard on the pedal, as you now have to make up for the lost assist from the vacuum booster. But that does not explain the shuddering either.

I once had a fuel leak that soaked the left front brake with Diesel fuel, making the brakes pull severely. It also introduced a shuddering. I have also soaked the brakes with water and made the pads seem useless until they dried out. This also introduced a shuddering that went away once the brakes were back to normal. Anyway, my point is I think the problem lies in the brake assemblies at each wheel, as the vacuum leak and master cylinder only address half the symptoms. Good Luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)