You need to either replace the float, or else get the gas out of it and patch the hole.
To drain/patch the float, I've had success doing this:
1 - Find the hole. Clean and dry the float, then warm it up a little (NOT with a flame!!). I simply hold it close to a 150 watt floodlight for a couple minutes. The leak should show up as a damp spot on the outside of the float - usually at a solder joint. Mark the leak.
2 - Get the rest of the gas out of the float. I simply left the spotlight shining on the float overnite (and OUTDOORS!). The heat evaporates the gas inside the float.
3 - Carefully clean and solder the leak. (I've heard of guys using epoxy instead of solder, but have never tried it myself...)
4 - Test the float: Put it in tin can and weight it down so it stays on the bottom of the can. Then put some gas in the can so the float is submerged. Leave the whole business overnite (the can covered and outside of course...) The next morning remove the float and shake it to see if any gas got inside. If the float is dry inside you should be all set.