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Front brake works fine. Rear brake does not.

I took the bike to the LBS to have the rear brake adjusted and get new pads. I expected them to work great, BUT, brake is still not stopping the wheel as it should. Bike shop guy said something about the pads have a "glaze" on them that has to be broken in by using them.

What other possible causes should I investigate ?

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    The pad break-in thing is true to an extent. Have you tried repeated braking to achieve this?
    – Argenti Apparatus
    Jun 26, 2019 at 19:14
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    Do note that the back brake will always have lower perceived braking "performance" than the front brake because of its positioning horizontally behind the center of mass. A back brake only stop will take longer than a front brake only stop, and is more likely to skid also decreasing braking performance.
    – Criggie ♦
    Jun 26, 2019 at 19:26
  • After 2 days of riding around in a parking lot (an empty one) the "break in" seems to be working and the rear brake is gaining effectiveness. Thanks to all who answered
    – Gus
    Jun 27, 2019 at 16:15

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Easy things to check:

  • Pad not aligned with the rim properly
  • Pads too far away from rim, requiring excessive lever travel before engagement
  • Excessive toe-out

You may also have too much flexibility in your brake cable and housing which limits the amount of braking force at the lever which is translated to the calipers. There's no obvious way to tell if this is happening, but if the housing and cable are old it's not expensive or difficult to replace them.

There a good park tool page on Canti brake adjustment, you can use this to see what the proper adjustment looks like.

https://www.parktool.com/blog/repair-help/cantilever-threaded-post-brake-service

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  • Old/hardened pads can cause poor performance too, and a straddle cable that is too long/short.
    – Criggie ♦
    Jun 26, 2019 at 19:27
  • @Criggie OP said they got new pads
    – Argenti Apparatus
    Jun 26, 2019 at 19:30
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    Old vs New cables can make a world of difference to how well brakes work
    – mattnz
    Jun 26, 2019 at 20:07
  • Alignment looks good (it was set by a professional). Pads are brand new and not too far from rim. I'm thinking flexibility in the cable (it's old) or straddle cable too long (it is longer than front wheel cable). I will try today or tomorrow to get on it and use the brakes repeatedly as per the suggestion. Tried it out today applying rear brake repeatedly. It seems better but still not what I would expect.
    – Gus
    Jun 27, 2019 at 1:26
  • @Gus How experienced a cyclist are you? Even a properly working rear brake only provides about a quarter of your total effective braking force, because braking naturally shifts your weight forwards, off the back wheel. If you're able to lock up the back wheel and skid it (never try to do that with the front!) then your brake pads and brake adjustment are basically as good as they'll ever be.
    – David Richerby
    Jun 27, 2019 at 10:15

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