An option is a low heat that would help the fuel in the float to evaporate. If you could get a heat source at about 100C and no flame that would be great. Now I think you are in Murcia Spain? Guess there would be plenty of sun at the moment. You could sit a piece of steel plate out in the sun with the float on it, the plate should get up very close to 100C. Once it is hot and the fuel starts to evaporate have a spray bottle with hot (so as to not chill the float instantly) soapy water and spray onto the float. Hopefully the evaporating fuel will create some bubbles so you can mark where the leak is. Then leave it to evaporate out all of the fuel before fixing.
