theres a bleed screw on the side of the fuel injection pump. Unscrew it a few turns, doesn't need to be all the way out. Then push the lever under the glass bowl until no more air bubbles. You might have a leak somewhere and air could be getting in.
A bubble in the glass bowl is common. If you have bled the fuel filter at the two bleed screws, bled the injection pump and she runs OK it is not a major concern.
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Brian