I have a leaking transmission on the 300SDL. The leak is coming from the rear servo(?) cover on passenger side. I got a new O-ring from my mechanic and replaced the one in the transmission.

All went well, or so I thought. Buttoned it up, drove it, and it is still leaking. Is there another O-ring/seal inside that I need to replace? Did I simply screw up the installation?

Another peculiar thing is that it only seems to push fluid out when driving. Don't see any drippage until it goes down the road.