I have used them on brake lines. I have a dual flare kit, but sometimes without a lift and restricted access to a specific area, it's almost impossible to flare a line that needs replacing. I spent 3 weekends trying to get the line right under the car. So I bought them and made them in 10 minutes. Some people might think this is bad for the brakes. Some people may say they are not sure. You know what I think is unsafe? Metal brake lines that will rust without warning, rendering your brakesโฆ