Update: a 30 year sweep told me my brush was too big and I finally get why
I was on a job in Springfield doing a standard 13x13 flue and the old timer who taught me years ago stopped by for coffee. He watched me pull out my 12 inch round brush and just shook his head, told me I was fighting the flue instead of cleaning it. He said a brush should glide with light contact, not jam up every two feet, and that oversizing like that is why I kept breaking rods. I dropped to a 10 inch and the same chimney took me 20 minutes instead of 45, with way less soot left in the smoke chamber. Anybody else here size their brush by the flue tile minus two inches, or do you match it exactly?
Man, my buddy ran into the exact same thing last season, swore by his 12 inch brush for years and couldn't figure out why his arms were toast by noon. Switched to a 10 and said it was like night and day on those 13x13s. I've been going two inches under on the tile since I started and it's worked out fine for me.