Ive had similar problems its the action of the brush that eventually loosens the cheap stick on lead beading however trad cleaning poses similar probs with scrims catching rough lead. I now tell all custys the pitfalls of the leaded glass what may happen and cant b held responsible. Most people accept, never had anyone trying to charge me 4 replacement
rather than scrubbing up and down work from the top as usual but move the brush left to right works a lot better than scrubbing as normal this way the bristles won't catch the lead as much