Are you in California?
Basically they crimp the vapor line at the canister and pressurize the tank with nitrogen. If the tank leaks, you fail. The places you can leak are the rubber joints along the vapor line. Where the metal vapor line enters the tank, the filler tube at the joint, the sender unit at the joint, the feeder line joint to the pump, or the sender gasket.
The problem then is to determine where the leak is. Or, how many leaks there are. You should take the car to a shop so they can pressurize the tank with smoke. That will let them know exactly where it's leaking. Then they send the tank out to be re-soldiered.
My daughters car failed the same test. The tank was leaking at the filler tube joint and the sending unit joint. We had to have the tank re-soldered. If I had the same situation today, I would have replaced the tank.
