So if this check valve wasn't working properly, this would cause excessive pressure build up in the tank, and extended "hissing" when you remove the gas cap?
I stopped somewhere in Wyoming earlier this week (heat of the day and driving at 80 mph) to fill up the 80 and the hissing and bubbling went on for a minute or more - so much so that I couldn't get the gas pump to put fuel into the tank. The vapors and gurgling kept shutting off the pump.
1976 FJ40 - green and mostly stock
1996 FZJ80 - not so stock