I have the same sled, and yes , with a tank filled to the top, the motion of the sled down the trail will bleed off a small amount of fuel from the breather tube, as you mention. There were threads about this long ago, no time to search now.
Look at your sled from between the top of the tank right by the steering column, there is a clear plastic tube, about 1/4 inch dia.
Follow it to the left where its strapped to the "firewall yoke". Dunno what to call it, LOL.
It just comes to an end, about 1 ft. above the chassis. Cut a couple ties, and route the end upward, careful not to kink it.
Thats what I remember about it. Hope it helps.