http://www.newton-equipment.com/tank_vent_valves.html

These valves are made for cars and will probably not work well in a
glider since on the road you are hardly going to find yourself going
up quickly (e.g. strong thermal - tank has to vent air to the
outside) AND not draining fuel from the tank towards the engine. While
flying the glider this could result in the valve being closed because
of turbulence (- fuel goes up the line and into the valve) and the
expanding air doing some damage to the system.

I'd try some cheap inline fuel filter mounted vertically so that the
fuel would have to go *up* through the filter to get out. If some fuel
gets to the filter it can retain a few cm^3 and i'd say there is a
good chance of the fuel getting eventually sucked back into the tank
if the filter located high enough. As an additional benefit you'd have
a barrier to insects or dust from the outside but it won't get clogged
more than the vent line itself.

Don't forget to ask for a Form One though...

On newer SH gliders the fuel vent line goes all the way to the top of
the vertical fin. Perhaps you could connect the vent line to an unused
ASI probe in the fin and reap the benefits of a slightly pressurized
fuel tank ! :-)