<<In other words, does it pass a similar amount of fuel as would be
passed if the check valve weren't there, just more slowly? Wouldn't you be
better off without the check valve and avoid all the failure modes
associated
with either pluggage of the two holes or failure of the check valve to
open?>>

I thought the whole deal with the check valves was to add a second vent in
parallel with the first one.  The second would presumably be vented to a
space probably at a lower pressure - such as inside the wingtip.  Then if
the first vent plugged the second would at least allow air to vent inward,
preventing fuel starvation.  Yes, I suppose if you had a plugged primary
vent the tank could still overpressure at high altitude.  Cessnas, I
believe, have a check valve in the fuel cap for that purpose.  I thought
that certificated planes had to have a redundant fuel tank vent capability.
No?

Gary Casey
ES project