On my 190e 2.6 I was just poking around in that area, there is a spring that assists the clutch pedal. If something is funny with that mechanism I could see it failing by holding the clutch pedal down. Also, on both my Peugeots there is a rubber hose that runs between the clutch slave and the chassis, then a steel line to the clutch master, that has gotten clogged and done strange things. I'm guessing Mercedes does the same thing so it could cause problems.

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5 speed '91 190E 2.6 320,000 mi. (new car, fast, smooth as silk six, couldn't find any more Peugeots)
5 speed '85 Peugeot 505 2.5l Turbo Diesel 266,000 mi. (old car, fast for a diesel, had 2 others)
5 speed '01 Jetta V6 (new wifes car, pretty quick)
5 speed '85 Peugeot 505 2.2l Turbo Gas 197,000 mi. (wifes car, faster, sadly gone just short of 200k )
5 speed '83 Yamaha 750 Maxim 14,000 mi. (fastest)
0 speed 4' x 8' 1800 lb Harbor Freight utility trailer (only as fast as what's pulling it)