I have a 1998 90HP Johnson Outboard. After I prime the bulb and it is solid, the engine starts and runs. However, if I shut it off for a few minutes, the bulb has air in it and needs to be re-primed. 18 gal external tank. Problem worsens as tank empties. Replaced fuel line to bulb and bulb. Is there an air leak somewhere after bulb? Or is it in fuel pump? Not a mechanic, but learning.