Hi all. I've seen old GB stems drilled for a centerpull brake; presumably at the factory. And i did one myself w/ no ill effects. You'd want to stay parallel to the quill and not too far out from the expander bolt so it lines up w/ the yoke cable. A smaller hole is probably better, say for a nippled ferrule rather than a bigger adjusting barrel. The GB had an adjusting barrel, but i don't remember if it was like a typical Weinmann [big] or the [small] one like came on the brazed on Raleigh rear housing stop. Good luck. Billy Ketchum; Chicago, IL; USA.