I found a leak in one by submerging it in near-boiling water quickly. The air inside will heat up quickly and start streaming out!! Once you find the hole, pull it out to cool, then re-submerge it with the hole facing down at the lowest point possible to expless and gas from the float. After several seconds pull it out to cool, then repeat, until a quick shake shows its empty. I repair them with a high power Weller soldering gun (250W, IIRC). Heat the area of the hole, apply solder, then slowly draw the gun across the filter, allowing the repair to cool but not the whole float, then pull the gun away once the repair hardens.