rollover valves are odd, they really dont do anything if you rollover, nothing closes, what happens is any liquid moving through it forces the valve close. Air does not.
Our problem is most of our tank vents are prone, when tank is full, to push liquid through the vent line when you brake to slow down. Thats why that plastic container behind the driver seat was invented, it was the rollover valve by making a place for fuel to slosh until it could drain back into the tank.
Thanks for delorean link i'm ordering another one, i had one ten years ago, really i did.