Tank does not fill up with air , air emptys quickly fills back up with air very slowly gauges never go to max

The big question is; when you think you have filled the fuel tank, does it have 80 litres (around 17.6 Imperial gallons, 21 US gallons) in it?

Fuel tanks shaped to fit an available space often have the filler pipe entering the tank far from the top so in order to fill the tank a vent hose runs from the extreme top of the tank to a point high in the filler pipe. Sometimes this hose becomes blocked and prevents the fuel tank being filled.

Perhaps your fuel gauge is fairly accurate after all and even if it isn't, a pessimistic fuel gauge isn't as inconvenient as an optimistic gauge...

the flue sense in your tank is bad most likely but it could also be just a bad connection between it and your flue gauge witch can be caused by corrosion or a lose wire

Sound like the float has a hole in it and fills with fuel when the tank fills up, you will have ti remove the gas tank and check the float, most of the time you can replace the unit as one.

how do you know one tank drops to zero unless you have a pressure gauge on both tanks sounds unlikely unless you are looking at the gauge on the pressure regulator

Sounds like the sending unit is bad in the tank.

the only thing that will cause this symptom is the fuel tank sending unit.

My 1999 GMC Sierra did the same thing. Fuel pump went out shortly thereafter.

THE GAUGE ON THE REGULATOR ONLY READS WHAT THE PRESSURE IS REGULATED TO NOT WHAT IS IN THE TANK. ITS VERY LIKELY YOUR REGULATOR IS BAD OR TURNED DOWN ALL THE WAY. YOU COULD EVEN HAVE YOUR QUICK CONNECT CLOGGED ON THE END OF THE AIR HOSE. MY MONEY IS ON THE REGULATOR THOUGH. TRY ADJUSTING THE REG. UP ALL THE WAY. IF THAT DOESNT WORK THEN DRAIN YOUR TANK TO ZERO, REMOVE THE REG. AND THEN TRY IT. REG. HAVE FLOW DIRECTION SO PAY ATTENTION TO THE ARROW WHEN INSTALLING. HOPE THIS HELPS, JODY

the fuel level sensor inside your gas tank might be defective
you need to drop the tank and check the level sensor

Scuba tanks can be filled well past 3200psi. When they do a hydrostatic test it is well beyond any pressure that any dive shop will fill a tank to. The highest I've seen the pressure get in my tank is 4000 and that is when the filler at the dive shop forgot about it.