Did they make absolutely certain that the vent hose coming out of the tank is not pinched? It is very easy to catch it between the tank mounting ears and the rubber biscuits on the frame when remounting the tank.
The effect will be the same.
If it really is the rollover valve, it is inserted into the tank vent hose in the general area of the center frame downtube. Take it out and shake it good until it rattles, then flush it out with gasoline or WD40. Let it sit until dry and shake it again to make sure it's not stuck.
If the tank is over filled and some gas goes down the small tube sticking up through the inner rim of the tank filler neck, it can close the rollover valve, and make it stick when it evaporates.
