65 Dart Help Please

If the float (brass) has developed a leak and sunk, you need to replace it. If it has sunk, it will be full of fuel, and you should be able to see the crack where the fuel got in. If there is no fuel inside the float (shake it and listen for the slosh) your problem lies elsewhere.

As for soldering a crack in a brass float, fuhgettaboutit. After you are positive you have let all the fuel inside evaporate, the second you play the torch on the crack you'll hear a remarkably loud "POP" and the little crack will become a wide open split that can never be repaired. Ask me how I know this!!!