Bad gas tank bladder?

That's not the bladder that's allowing or not allowing that The bladder is just there on top of the tank to help control fumes and outgassing of said fumes and vapors into the atmosphere The check valves and associated plumbing could be fubar and any of those valves that are closed when they should be open and so forth will cause this low filling The blinking business is possibly not related and is you're sending unit for the gas gauge they can blink for numerous reasons I'm guessing you're 12 volt and everything else is perfect so on the six and a half gallons you're going whatever the mileage you said above? Usually when this has happened to me and two of these cars I got under the car with a rubber mallet and banged on the bottom of the tank all around the bottom of the tank boom boom boom we're not trying to put huge dents in the tank or any dents in the tank or anything like that just trying to dislodge any garbage or crap that may be on the slider or whatever works the gas gauge float mechanism

 

you'll never know if you filled up too soon, or ran out too soon.