I had similiar symptoms a year or so ago. You don't mention whether or not you found any hydro oil on the outside of the resevoir or on inner fender/metal work of the car.

I ask because I once mistook a leaky SLS with a waterpump going bad. Seems hard to to do, I know. Both fluids are the same-ish color and it was hard to figure out what it was (or even if it came from my car) when not a fresh spill.

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1982 240 D, 308,000 - 321,127 miles (sold)
1982 300 TD,166,500 - 226,000 miles
1998 E 320, 120,000 - 144,000 miles
2005 C 230 K, 26,000 - 77,000 miles (sold)