Maybe air trap.
FWIW
For me, previous fuel filter changes have had no problems, but the last filter change, the primer bulb would not come up firm. Was cranking with main battery and was quickly fading so paralleled up Aux. Only a couple attempts and decided not good.
Ended up removing both fuel tank filler caps and primed up by leaving larger filter metal outlet hose off and squeezed primer bulb letting the filter vent thru outlet pipe, then placed finger over the metal outlet pipe while letting the bulb expand to draw fuel into and thru filter (many pumps). Pressure in bulb came up when the filter was near full and when I had a visual of fuel put hose on, primed bulb further maybe 6 times and came solid. Glowed / cranked and eng started straight up and then pumped primer a few more times at idle and all was good there after..
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