Regarding the latest discussion on tank vent check-valves.

FWIW:

When I started flying my LNC2, I had just open aluminum tubing vents
sticking out of the bottom of the wings and fuselage (for the header tank)
with the bottoms filed to about a 45* angle facing forward. While
transferring fuel from the left wing in flight, I noticed that I could begin
to see a slight deformation in the top of the wing (the light was just
right, I was lucky). I stopped the transfer and landed to check it out. I
found a spider had taken up residence in the vent, just about completely
plugging the tube. My solution was to Tee into each vent tube and add a
check valve inside the airframe, with the open end covered with a ball of
several layers of cheese cloth (to keep the bugs out but not restrict
air-flow). I then tested the valves by closing off the open vents and
pumping fuel from each tank - problem solved.

My check valves are not there to keep an over-filled tank, uneven parking or
thermal expansion of the fuel from dripping out the vents, I've designed my
vent plumbing to preclude that so don't have the problem. What I have is a
back-up vent system (admittedly that doesn't provide any pressure recovery)
so I don't have to worry about a plugged external vent. Once was enough.

Dan Schaefer