It is actually typical to have to replace both master and slave. The clutch circuit is fed by the rear of brake fluid reservoir, but the clutch typically lasts the life of the engine, so the fluid is normally never changed. The two cylinders sit at the dead end of the hydraulic circuit and therefore spend the bulk of their operating lives in degrading fluid. Once one begins to rust and cause leakage, the other is soon to follow. So, replacing both is a good practice and often necessary.

I have had the steel tube fail as well in the past and drove from the NY side of Lake Champlain 300 miles, towing a jet ski, with no clutch, back to CT due to a fatigue crack in that component. Granted most of the driving was on the highway. Shifting the 240D without a clutch is not that bad. Downshifting can be a challenge, but I only had to shut down at toll booths.

Also, I have never had to do any tricks to refill the clutch lines. Just fill and pump. Takes time but eventually the fluid line goes "solid" with fluid and clutch behaves normally.

Jim

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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)