Thanks. To clarify, you installed a rollover valve in the vent line on your midget (that was itself plumbed into the filler neck? As for leakage per Nate's comment, I guess I'll remove the filler neck, invert it, put the cap on and fill it with liquid to see if the cap actually seals it completly. Assuming it does, then no rollover valve of any kind would be required.
If it doesn't then a vent valve isn't a total fix. In that case what do others with OEM fuel tanks do to positively prevent leakage? As an aside my Spec Miata had a factory ball valve in the neck to seal the tank in the event of a rollover. BTW, I know that works since that car once spent ten minutes on its lid with nary a drop leaking!
Rick