[mark786]

Gareth,
You can't count on the check valves working correctly all the time so don't use them.
I used to use a two line tank but gave up and now use three.
With two you can just put a tee in the carb. line for the fill. On a nitro motor this would flood the carb but the way the carb works on a gas motor it would be OK.
Vic's explanation is a good one.
If the vent line is looped behind the tank you will get very little spillage during flight. The vent line should exit the plane about an inch below the fuse or cowl.
I only have to clean the bottom of my plane about every twenty flights. Even then it isn't dirty or covered with fuel.
Mark