Brush or drill on a 60-year-old flue in Gary, and I picked wrong

Got called out to a brick ranch in Gary two weeks ago and the homeowner swore the flue had never been touched since 1963. I stood on that roof for ten minutes going back and forth between my stiff poly brush and the drill whip, and I went with the drill because I figured the old soot would be baked on hard. Bad call. The whip chewed through a soft spot in the mortar about four feet down and I watched a chunk the size of my fist drop into the cleanout, and the homeowner heard it from the kitchen. Ended up pulling the brush through the rest by hand and it went smooth as anything, which stung a little. Any of you got a rule for telling soft old mortar from solid before you pick a tool?