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not sure if these cats have it but prius since about 2004 had a gas tank with a soft plastic bladder that collapses as the fuel gets pumped to the engine so as to have as little air for exhaust fumes to collect. over time they will sometimes get too rigid and won't allow the tank to fill well. i really doubt this is the case but i will poke around on TIS tomorrow when I'm at work to see if i can't find any info. vent valve could also cause this if its stuck closed. they are normally open any time the car is off. could be causing an air lock in the filler neck causing a fuel backup. that usually means replacing the entire evap canister assembly to the tune of a million dollars. if you don't get a check engine light soon though its gonna be a hard one to figure out