I would say it is one of the two valves in the link below.
As read the text about pressure in the tank.

1996 Bronco/F-Series

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I replaced this:


Do I even have one of these?

 

i would say to get a bottle of soapy water and do your think and then spray all your evap system a little at a time and look for bubbles cuz its venting somewhere i remember seeing the pressure warning by the gas cap and on some vehicles is normal.but fact is its leaking and its got to be happening from the cab forward most likely to be drawn into the air ven on the cowl

 

I pulled the fume vent line off of the rear tank and it had raw fuel in it. I am assuming that the check valve has failed. Does anyone have a part number or a place to find one?