I would think it would have to be the accumulator on that side. could be the strut but less likely, if it was leaking at the top you would see it running down the sides. im not sure 12.5 is really that low, 13.5 seems like its on the high end. you didnt use your lever to set the ride height did you ?

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1996 E300 110,000
1985 300TD 212,000
1980 300D 238,000