My 77 450sl did the same thing. Turned out to be the reverse band servo piston seal. It is located behind a round cover on the left hand side of the XMSN. All I did was pop the retaining spring out and the cover along with the piston popped out. You may want to drain fluid first or you'll get a good bath. Just remove the seal if it's bad and install a new one on the piston(It's just a rubber ring and reinstall. You should be able to get the seal from any dealer fairly inexpensively.