If you have an oil leaking from the front of the transmission ,you will have to rebuild ,or replace oil pump.New pump costs around $300-$400,rebuild-gaskets around $130.
1. Remove transmission http://www.benzworld.org/forums/w140-s-class/1368666-transmission-removal-m104_s320.html
2.remove pump bolts,note that theres two vacant holes on each side.
3.Screw two bolts into threaded holes and use them to pull out front pump.
Clean surfaces,replace pump gasket,do not use any sealant.Use non hardening sealant and torque bolts to 13Nm.
4. Remove two Teflon rings.
Insert two new Teflon rings with TRANSMISSION ASSEMBLY GREASE.
Assure that the ring gaps remain together.
5.To remove spring plate and springs(total number 20) ,you will need to make a tool.
I used 3" in diameter pipe(from your local plumbing store-Home depot),and later i did cut a window to get access to a snap ring,12" bolt with washers and nut, and any kind of metal plate (drill a hole for bolt).
6.Clamp,until snap ring is exposed,remove snap ring ,remove plate and springs.
I found that the best DIY'er friends are white paint-when disassembling something,to mark positions,and zip bags,to keep bolts,nuts,washers ;
in this case springs from ....
7.Mark and pull out B3 piston.
Replace Lip seals,apply oil to the seals,make sure that the lip pointing towards the rear.
When inserting piston make sure pin and the hole coincide,make sure not to crimp seals,when installing piston.
8.Remove pump front cover.
tighten bolts to 20NM
9.Top and bottom of pump gears differs,make sure you reinstall them right ,bevel points towards radial seal.
Install new radial seal(red seal in the first picture-front pump seal)
10 Replace pump O-ring by installing it into the groove.
Good luck Vladimir.