Bubbles in that fuel line are undesirable. Maybe you have an air leak into your fuel system. Search around here and you will find good advice on getting air bubbles out of the fuel system. If they are re-appearing, you must have a loose connection or cracked fuel line.

Many on here recommend a new type of primer pump. If your primer pump has white-ish plastic as the part you unscrew, it is the old type. I guess they leak air which is why many suggest you get rid of it. I still have my old type primer pump, but I also have an unsteady idle which might be related.

The fuel tank ventilation system is hiding behind the spare tire (right side of the trunk). It is a plastic box with 3 small hoses going to it. It vents to a rubber port behind the back bumper, at the corner bend. I guess fuel tanks can implode if the ventilation system isn't working (hopefully this is a rare event).

You should probably try to track down the MB service manual for your car. You can find the link through this forum, and electronic versions are quite affordable. It has been really helpful for me to learn more about my recent acquisition.