I did just that; replaced the o-ring but it's still leaking. I am not seeing it leak from where the o-ring sits between the reservoir and pump. The hose connecting to the reservoir to the radiator was replaced too and it's dry. The reservoir is not leaking either. But what I am seeing is a small leak that is coming behind the pump pully. For good meassure I checked if anything above was leaking (valve covers) and thankfully nothing is. It seems it only leaks when the engine is running. So I figure some seal in the pump has finally given up? I saw that some S and R class owners have rebuilt their pumps and i figured I could do that.More than likely it is the faulty O-ring on your reservoir that is causing the leak.