12

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

Improve this question
3
  • Is any brake fluid leaking?
    – DWGKNZ
    Mar 31, 2014 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, 2014 at 19:51
  • 1
    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, 2014 at 11:49

4 Answers 4

Reset to default
10

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.

Improve this answer
5
  • i'll need to get some oil but I'll give that a try and report back. thanks.
    – Alex
    Mar 31, 2014 at 19:53
  • 1
    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, 2014 at 19:56
  • 1
    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, 2014 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, 2014 at 19:43
  • 2
    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, 2014 at 0:30
7

If the steps suggested by @DWGKNZ don't work you should consider taking it apart and inspect the Piston seals. Over time the piston seals can dry up and get gummed up with a little dirt and what looks like limestone.

Overhauling most hydraulic brake systems is doable by the home mechanic and Avid sells a parts kit for the Juicy 7's.

After I rebuilt mine I noticed the pistons still didn't retract evenly. One side would move more then the other. The reason for this is the friction between the piston and the piston seal. Dot fluid will help lubricate the retraction movement but that alone may not be enough. Ideally as part of the overhaul you should use some form of silicon grease. Avid sells a compound called DOT GREASE that is specifically designed for piston seals and O rings.

You also need a bleed kit if your are going to rebuild.

  • Take the pads off and clean the area with brake cleaner.
  • Pump the lever to push the pistons out a 4-5 mm and dab dot grease around the piston edge with a cotton wool bud or very small paint brush.
  • Push the piston back in and repeat the process a few times, forcing the grease into the piston seal surface.

Eventually it should be nicely lubricated and pistons should retract quickly and evenly.

Obviously it would be easier to just buy a new set of brakes but I personally learned a great deal by doing it myself and found the whole process very rewarding when you finally fix it.

Improve this answer
3
  • 2
    Welcome to Bicycles! Instead of a typical forum, this is a Question & Answer site. As such we are looking for focused and factual answers. You should note that this question already has an accepted answer that is a good example of what we are looking for. If you agree with it, rather than adding a new answer laced with anecdotes, you should upvote the existing answer. Please see the Tour for an overview of how this and other Stack Exchange sites work.
    – Gary.Ray ♦
    Mar 18, 2017 at 4:45
  • 1
    Hi, What was trying to add was my experience of the dirt build up in the piston seal. This hard stubborn dirt you will never actually see unless you take it apart. I didn't feel the accepted answer addressed this point and I have also never seen this mentioned in any other forum so I thought it may be helpful for others experiencing the same problems. As my juicy 7 brakes were one year older than Alex's I feels is very possible we had the same problem. If you feel my answer adds no value then please feel free to remove or moderate it. I was just trying to help the community with my experience.
    – Btwin Fan
    Mar 18, 2017 at 11:17
  • 1
    You have a great point, and your answer wasn't bad (though sometimes anecdotes get in the way of a clear answer). Check out the formatting options available using the icons above the answer box. Formatting can make what you write much easier for others to read. I'll make an attempt at editing you answer; check it out as a guide, and make sure I have retained your meaning.
    – Gary.Ray ♦
    Mar 18, 2017 at 22:10
4

I had the same issue -- brake pistons not retracting. Issue ended up being "crumbled" piston -- the undersurface of the piston had broken in to small pieces. Here are some photos:

One large piston fragment is visible in the caliper cavity. The piston is seen at the top, in profile

Right caliper and piston shows crumbled fragments.

[Again, piston and caliper showing crumbled fragments3

Improve this answer
0

Your brakes are of a good brand. You should try bleeding the brakes first before binning them and buying new ones. Hopefully this link should help you.

Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.