B2 piston is responsible for forward, but the $5 seal(black seal) is generally the culprit

http://www.dieselgiant.com/Mercedesb2pistonsealreplacementl.htm

Reverse is the lip seal right in front inside of the transmission-See picture 7(the small seal)

http://www.benzworld.org/forums/w140-s-class/1369461-722-5-transmission-oil-pump-rebuid.html

Martin