Rear brake tension/spring problem
 

Recently I replace the brake pad on Tektro R540 (dual-pivot sidepull brake caliper) for my road bike. I followed the instructions with Zinn's road bike maintenance book, but I have some problems with the rear brake.

First I tried adjusting the barrel adjuster (82), but since the new brake pads are thicker than the old used ones. I loosen the cable fixing bolt (70) to readjust the tension. After everything seems ok, I tighten the cable fixing bolt. But then once I hit the brake, the tension became loose again, and I can see the brake is almost rubbing the rim until I use my hand to open the brakes myself, (as if the spring doesn't work). I try this a couple times and still couldn't solve the problem.

I bought this bike last year, and I think the spring should still be good. The front brake is still pretty good too. I checked the cable, doesn't seems to be damaged neither. I am not sure what else to check. Please let me know you know what the problem might be and how I could fix it. I would like to try to fix this myself before I go ask help at the LBS.

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