If you can't find the leak, heat up some water in a pot, drop in the float, push it underwater with a piece of wood, and see where the bubbles come from. (The air in the float will expand due to the heat, and you will get a stream of bubbles. Mark where they come from immediately.)
I think you'd have to get a replacement float from someone who keeps Villiers bits. There are some Outdoorking members who do so.
