Recently purchased 'thru gas cap fill cap ' that replaces original gas cap on my gen II 2008 Prius. First couple of fills were ok. Only slight movement to get filler nozzle thru both the gas cap hole and the original fill hole. On subsequent fill when tank was almost empty, there was sudden pressurized spray from fill cap while I was still squeezing the gas nozzle. Gas hit me in face and shirt. I wouldn't want this to happen again so I put original gas cap back on and tossed the no-remove gas cap.
I think the bladder is involved in this spray back, because spray continued after releasing fill lever.
The gas cap is part of the Fuel Evap system. If it leaks, it will show up as an error as a Check Engine light and an OBDII error code.