I purchased the Jagwire MTB replacement cables from Performance Bike shop about 3 years ago. It was a chain bike store that closed up shop in ABQ, NM, in 2019. I can't remember the exact cable model I purchased or the price; but, it needs to be for MTB ready because of the barrel ends like "E4c" that secure withing the Rad brake handles (assuming they haven't changed since 2016).
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I just did red outer end caps crimps tips ferrules (60 for $9) and a crimp tool ($12) on Amazon.
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This is what my Rad brake cable looked like once I started to break strands from all my brake adjustments. The broken strands started to bunch up within the brake handle and acted like a spring. The brake handle wouldn't return to the normal position sometimes and engaged the motor cut off. All those strands were squeezed into a space between the barrel end and brake cable tension adjustment knob on the handle.
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