The primary filter will trap dissolved gas from your tank. Fuel is cool and under slight pressure by design. This is ideal for dissolving gas. When the fuel gets up to your engine it heats up and de-gasses. The primary fuel filter is designed to trap gas. That is why the outlet is low. You should not see a continuous stream of bubbles coming into the filter, but even if you get rid of all of the gas in the filter it will come back eventually.

I have seen this myself and I am sure I don't have a fuel leak before the filter.

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1991 350SD (updated rods)
Biodiesel B100 when I can find it.
Dino when really cold outside