Well here's the scoop...

Before I had a non functioning sending unit in my fuel tanks ('88 F150 5.0/AOD). I swapped them out recently.

Front tank works fine. Gauge works nicely and pumps fuel. No spillage.

Rear tank...I fill it up and it spills everywhere. (gauge works in the dash now finally and pumps fuel) I checked that retaining ring and that's fine. That vapor hose is in place too. It's not coming from the filler neck but I can assume that some sort of check valve in the tank isn't working and it just keeps filling and spills out the top of the tank by that retaining ring.

Could it be the entire sending unit I have is bad?

Is there supposed to be a gasket there at the top?

Do I/will I need a new tank?

What would cause this to just keep filling?

Please help, I really don't wanna pay someone to fix this.