Gas sending unit float.

I've repaired a few floats with JB weld. You can empty the float by warming it with hot water (or carefully with a heat gun) and watching for where the liquid dribbles out, it may take a few cycles.  A afternoon in the sun will make sure the float is as dry possible. Once you know where the hole is, clean that area really well with some emery and wipe down with acetone.  Mix up some JB (thinning a bit with a little acetone will help the next step). Get the float nice and hot with a heat gun. While it's still hot, spread the JB over the area with the hole(s).  As the float cools, a slight vacuum will form, pulling enough of the epoxy into the hole to ensure a good seal.

Plain solder works too, once the float is cleaned out. I've done it by torch (flux) and soldering iron (plain rosin core electrical). JB was a little less hassle...

Check your work by holding the float under a sinkful of hot water, it there' still a pinhole, you should see bubbles form.  Repeat till it's sealed.  YMMV