I found out that a 190e doesn't have a mono valve. It's a heater valve and it appears that the leak is coming from the bottom of it. There might be a crack in the casing or it could just be a hose leaking. Anyway... it's a very slow leak but should be easy enough to replace. It's right underneath the air intake on the passenger side behind the battery. I'm asuming it's the heater valve. Does this sound right?

Your heater valve is under passenger side lower of the winshield 000-830-85-84 is the parts number.

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MB STARS Master Guild Technician,17 years MB tech
14 years independent shop owner