a check valve is pretty much just a flap inside tubing. It allows flow one way, but not the other. When a fluid tries to reverse its flow, the valve shuts and doesnt allow it to pass. I'm pretty sure that the filler neck (the tubing that goes from your gas cap to your tank) has a check valve that only allows the gas to enter the tank, and not go back up the tubing. Your valve however, could be stuck open, thus allowing the gas to flow up the filler neck and into the gas cap area.
To remove the neck, you will need to remove the 3 bolts from around the gas cap area and also drop the fuel tank. The tank has 2 straps holding it up held by 2 (1/2" i believe) bolts. Make sure you run the thing to close to empty (because a full gas tank is heavy
) and then get a friend to give you a hand in lowering it. You won't need to disconnect any lines or connections to get the filler neck. But it is a little bit of a pain to get the neck out of the tank. You just have to snake it out of there, then see whats goin on. You may be able to lubricate it, or get one from a junk yard.
Are you sure it only happends when its less than 3/4 of a tank? that seems somewhat odd.