You might want to check and see if it's leaking around the rubber washer/gasket that the washer motor mounts through - these do get hard and begin leaking after many years. They are very cheap to replace. The newer reservoirs are different in that they have a second motor mount area molded into the case design to accommodate a second washer motor for headlight washers, but will work fine in older SLs, my '82 has one. If you get a newer style you must also get a rubber plug for the second washer motor mount hole.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C