Erick,
There is a rubber 'o' ring under the alli washer and also a seal (not unlike a valve stem seal in shape) in the housing that you undo to change the washer.
I trust you have read the topic Joe reffered you to. While you are there, its worth checking the 'lift' in the drive gear and shimming it if necessary.
part #s:
126 990 0140 alli washer
621 997 0040 o ring
007 997 7346 seal
naj
'Kloines Scheisserle'
65 230SL
68 280SL