Hi
I also find this happens when using a double trim sill brush , it's good on flat glass, but have to be so carefully on leaded, I put it down to the amount of splay in Brissells , slicing of lead , has happened to me a few times since using this brush, never happened with my old single trim supalite brushes , with very little splay, don't get me wrong , the dual trim sill brush is excellent , just gotta be carefully on leaded, or use a different brush
Daz