[QUOTE=Beagle;2857589]It is perfectly normal for a bubble of air to be trapped in the pre-filter – in reality it is not trapped there. Air is entering the primary filter continuously as it is flushed out of the pre-filter and the Primary filter is designed to cope with it. The source of the air is fuel foaming in the tank as it sloshes around - ever filled your tank right to the top and had to wait for all the lather to settle?

Once the fuel system has been fully bled it is impossible for air entering the Primary filter to reach the Fuel Pump L/P chamber unless:
a) The filters, tank strainer or fuel lines are blocked
b).[/QUOTEc

I do not believe the comments in the fist paragraph. If you connect a longer Fuel Inlet Hose and run it into a separated container of Diesel Fuel (with no air Bubbles in it) you will find your Bubble still in the Plastic Filter.

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84 300D, 82 Volvo 244Gl Diesel