Air leak - Racor 500 fuel fiter
There are lots of fun ways in which air can enter your fuel system - this was a new one to me:
The sailing yacht in the slip next to mine has the fuel tank built into the keel with the engine mounted above it. A Racor #500 primary filter is located level with the top of the engine. This is connected to the engine's lift pump. After running under load for a few minutes the engine would stop and could not be re-started using the plunger on the lift pump. The Racor filter would be half empty. There was plenty of fuel in the tank and the draw-tube was not blocked.
Here's what it turned out to be. This is the opposite of a gravity-fed system: the lift-pump sucks fuel from the tank through the Racor, thus the Racor is under vacuum and air will enter anywhere it can. In this case the little O-ring (green arrow) had become hard with age and was no longer making a good seal. A tiny stream of bubbles could be seen rising through the fuel in the bowl.
My own Racor drain valves look like the one on the left which use a different type of seal (pink arrow). IMHO, the one on the right is a nicer piece of engineering.
If all else fails, read the instructions
If it ain't broke, don't fix it