Hello all'

My 1992 240 recently developed a coolant leak where the heater return line connects to the water pump. I replaced the water pump with a HEPU one and the leak is still there. I noticed some slight rust on the tip of the metal line but thought the O ring would be able to seal it.

The leak tends to go away when the engine is hot and once it cools down the leak comes back; so my suspicion is that the O ring expands when hot and shrinks when cool, thus the leak returns.

Is this common? I wonder what are my options here, your advise is appreciated.
Jorge