Well, like I said, I'm still waffling on weighting the handle.
I'll probably finish the restore and use it before I decide. This one will be easy to weight after the fact if I choose to.
In my experience though, putting a fresh, dry, hollow handled brush into a sink of water floats the entire brush not just the handle. At least until you get the knot wet enough to sink. And once the knot is wet enough to sink, it will whether the handle is heavier or lighter than it is.
In fact, it was using a hollow handled brush that got me switch to a bowl to soak the brush quite some time ago.