generally, any valve or device located part way down the filler neck is to prevent stealing gasoline or prevent large objects from entering the neck. I'm assuming your talking about the little tin flapper that the gas nozzle pushes down as you insert it to fill your tank. any important part of the gas system would be in the tank itself or in the gas cap. I'm not a prius expert just sharing my general knowledge. Hope that helps.
Toyota Prius was created in 1997.
The Prius was invented and developed by Toyota.
Camry Prius C Prius V Volt
Toyota Prius v was created in 2011.
The 2001 Toyota Prius has 16 valves.
The 2011 Toyota Prius has 16 valves.
The 2010 Toyota Prius has 16 valves.
The 2002 Toyota Prius has 16 valves.
The 2006 Toyota Prius has 16 valves.
The 2005 Toyota Prius has 16 valves.
Bugs Bunny does drive a Toyota Prius.
The 2013 Toyota Prius has 16 valves.