Sorry to hear about your redundant training in water pump R&R-
Check to see if the pump mating surface is really flat. Try using a steel edge ruler, moving all over the surface. Look carefully for any light gaps.
Is it the correct O-ring? If you still have the old O-ring match it up. Compare for the obvious overall diameter but also thickness.
Where did you get it and the pump?
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)