If the float is brass, it is normal practice to remove the fuel from inside it, then solder it up. Have you found the leak?
The process is to warm up the float so the fuel will evaporate. Do not use a direct flame, or the plume of petrol vapour coming out of the float will ignite. You also need to control the pressure inside the float, which implies using only a small heat source. Of course an alternative is to drill a small hole in the float to facilitate rapid draining, then solder up the hole after it has been held at say 100 degrees C for half an hour, but this will leave the float looking repaired.
