I think we all have a bubble in our Plastic Primary Filters. As long as the Air is not being pulled into the Fuel Injection Pump the Bubble is not an issue.

If you have no Hand Primer on your Fuel Supply/Lift Pump there should be no source of Air leak in the Fuel Supply Pump. This means any Air Leaks would be down stream from the Fuel Supply Lift Pump.

Also if you have no Hand Primer you Fuel Injection Pump is setup to allow small amounts of Air out of the Fuel Injection Pump Housing by way of the Fuel Pressure Relief/Overflow Valve.

However, In the DIY section there is some instuctions on how to rebuild the older and newer Fuel Supply/Lift Pumps.
DIY Repair Links
DIY Links by Parts Category - PeachParts Mercedes ShopForum

PeachPartsWiki: Do It Yourself Articles - Mercedes Vehicles

__________________
84 300D, 82 Volvo 244Gl Diesel