Mark,

In my 280 hours of flying my 360 I've spit about a pint of fuel on
various ramps.  This only occcurs when topped off and inadvertantly
turning on the ground at relatively high taxi speeds.  I therefore am
happy without check valves.  Operationally, once topped off, I transfer
fuel almost immediately after takeoff to minimize any uncoordinated
flight potential leakage.

However,  If you must eliminate any ramp fuel leakage you might consider
alternatives I've seen from other builders.

1) The RV world takes the vent into the cockpit to allow a vertical leg
of 2-3 feet.  This  head pressure eliminates any outflow through the
vents.

2)  One or two lancairs have placed the vent in the wheel well.  Theory
here is that while high speed cornering the fuel wants to travel
outboard and isn't allowed to go streaming out the vent.

My biggest hesitation regarding a check valve is future failure
potential.  It's one more item possibly requiring accessibility to
maintain.  Additionally, it's another one of those 1/4 kt items I felt
I'd be happy without.  It should be noted however that I'm a 1/4 kt
freak.

Larry Henney
360LH