1997 Mercedes Benz C250 Mercedes C250 TurboDiesel 1997 air

Dear Friends,

I have a Mercedes C250 TurboDiesel 1997.
The car does not start after parking it for 1 or 2 days. I also observed air bubbles in the wires around the fuel filter.

The mechanic worked on this problem for a long time and changed the fuel tank, every plastic pipes going from the fuel tank to the engine but the problem has occured again last week. The mechanic came again and discovered that the fuel was draining back to the fuel tank when parked.

Now, our issue is to find what causes for the fuel to go back to the tank: he thinks that it might be a valve problem in the "thing" that maintains the fuel filter.

Somehow, even after changing all the pipes why is there still air in them?
what is the cause for the fuel to drain back to the tank?

P.S: I think the problem occurs when the weather is cold ( around 5 degrees)

Thanks in advance.