If you use a new horseshoe gasket and the Mercedes Black sealer, it will not leak when done. I have done dozens with good results.
The thing I have found with persistant leaks is that the head gasket or the lower front cover is leaking. If this is the case, you will only be able to repair by removing the front lower cover or replacing the head gasket or both.
I have done a number of these under warranty and the car comes in 10k mile later BONE DRY .... Like a HONDA under the bottom.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS