[Steeveedee]

Crappy solder joint would be my take. When you solder the float pieces together, there will be low pressure inside as the parts cool. That can suck solder in, causing a leak. Maybe try a bead of JB Weld? I had a plastic one from a Buick that I sold. I ended up using that after one repair attempt on the brass float. I didn't think of the JB Weld at that time.