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My front disk brake's piston doesn't fully retract. While I can ride the bike, the brake is slightly rubbing. (IE, if I just spin the wheel with my hand, it'll go around about one and a half times and then stop.) It's definitely slowing me down.

If this fixable?

The advice I got was that it's not worth the trouble and to buy a replacement set. The current brakes are Avid Juicy 7s that are about 7 years old

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Is any brake fluid leaking? –  DWGKNZ Mar 31 at 17:12
    
not currently (there was some leakage at the reservoir next to the handle a year ago but it cleared up.) –  Alex Mar 31 at 19:51
    
this didn't go well. i snapped off one of the knobs inside the brakes as I was trying to put the brake pads back on. –  Alex May 9 at 11:49
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1 Answer 1

up vote 2 down vote accepted

I would try repairing them before binning them and purchasing the Shimano upgrade. If there is no obvious fluid leak around the piston the seals are likely in good order and you've got a sticky piston rather than a failed calliper. This is a simple fix:

  1. Remove wheel and brake pads.
  2. Using a flat headed item hold the working piston in.
  3. Squeeze the lever, the non-retracting piston should be fully out.
  4. While holding the lever wipe around the piston with a clean lint free cloth.
  5. Still holding lever lubricate the retracting bit of the piston with a bit of brake fluid (DOT 4 or 5.1, not oil, as this may damage the seals and/or might work through the seals and mix with the brake fluid)
  6. Release the lever and repeat lubricating 5 more times, the piston should now be retracting.
  7. Wipe off any residual brake fluid from the caliper.
  8. Use a brake block to push both pistons evenly back into the caliper. Pull the lever with the block in to align.
  9. Refit the pads and wheel.
  10. Realign the caliper.

Please note, never pull the lever when there's nothing in between the two pistons. It's hard to separate pads and near impossible to separate pistons without bleeding.

Juicy 7s were good brakes, if they aren't broken there's no point rushing out to replace them.

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i'll need to get some oil but I'll give that a try and report back. thanks. –  Alex Mar 31 at 19:53
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Needs to be DOT 4 or 5.1 brake fluid, not normal oil as that could work past the seals and contaminate the fluid. Is cheap at automotive stores. –  DWGKNZ Mar 31 at 19:56
    
Also, take care not to pop a piston right out of the calliper. Easy done on Avids, don't ask how I know, ahem. –  alex Apr 2 at 1:07
    
@alex, not seen that but I imagine it is a PITA, can you drain the system and stick the piston back in? –  DWGKNZ Apr 2 at 19:43
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It drains itself pretty well when it's missing a piston, I just pushed it back in and refilled. You're not really meant to do that, but it's been fine since. Touchwood. –  alex Apr 3 at 0:30
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