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I have a bike (Cannondale Quick MX) I used frequently until last fall, when I got a driving license (not my first, well out of my teens) and a car to use. The bike is 3 1/2 years old now.

Since the car ruined my life ;) I only ride occasionally. Recently I got it out for the first time in a month or so :( and discovered the rear brake had no pressure -- the handle was loose and the calipers did not move.

I got a bleed kit a while back when I felt the pressure was low, and a litre of fluid, so I refilled it and it is now working normally. While I was doing this, at one point when pumping the brake handle with the bleed valve closed (following instructions, doing this alternating with quickly opening the valve to let air bubbles out) the caliper1 popped out (I had removed the pads2).

I put it back and applied one of the plastic tools in the bleed kit (there's a bunch of these that aren't mentioned in the instructions) that clearly seemed intended to insert into the space where the pads go to hold the calipers in place, then at the end put the pads back.

However, the brake itself is slightly too tight -- not noticeable riding, but if you spin the wheel freely with the brake open, it stops just a bit prematurely because the disc lightly touches one of the pads, pictured below.

enter image description here

Notice the top pad is much more worn (that's not just a trick of the light), such that toward the neck of the clip it is down to the clip itself.

I guess I have to replace these, but do I also have to replace the brake? It's obviously been doing this for a while. Have I put in too much oil (I don't really see how that is possible, but...)? The handle is currently a bit soft (much softer than the other, identical one). Basically this reminds me of a sticky or misaligned V brake, but there doesn't seem to be anything I can tweak with this. Reducing the pressure would make it unusable, and I don't even think it would help the actual problem.

  1. Pretty sure that's what it's called anyway; I tried looking around for detailed diagrams of the brake but they did not label the caliper specifically. Anyway, a nickel sized shallow piston that sits freely inside the brake.

  2. One set of instructions online said to do this; the ones that came with the bleed kit just said to remove the cotter pin, which all by itself seemed pointless so I presumed "then remove the pads" was missing...

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The part that popped out is one of the pistons.

There should always be a bleed block in place between the pistons when bleeding the brake. Immobilizing them this way is what prevents overfill and also prevents the pistons from popping out if you go to squeeze the lever as part of part of the bleed procedure, both of which happened here.

Excessively narrow pad gap is the main symptom of overfill.

Bleed them again with the block in place and everything should be good.

The piston that came out probably went back in clean and lubricated, while the other one remained however it was. That kind of thing can cause uneven pad wear and adjustment weirdness, as one piston moves more readily than the other. You could try addressing it by intentionally overextending each piston in turn and then cleaning them with a cotton swab and alcohol. Removing them all the way isn't necessary.

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To add to other answers, there is a great video by Park Tool published recently, explaining the hydraulic brake pistons. They explain accidentally blowing them out as well as their cleaning process.

Prevent "sticky pistons" by cleaning and lubing the pistons every time you change pads. We talk a bit about how the brake functions and walk through the cleaning procedure to help your brakes in the long run.

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  • 1
    Part of my sin here is that I hate watching online videos to learn -- which if I search for bike repair topics that's mostly what I find. So I really did not do much research. Guess I have to buckle down, sit still, and focus ;)
    – goldilocks
    Aug 20, 2020 at 20:20
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    I understand and share your aversion to videos. I prefer a properly written text with illustrations to any video on the same topic. A text is much more compact, it is much faster to understand whether it covers a problem or not, it is much easier to jump back and forth between its sections compared to scrolling a clip. But this is the reality we are living in now, and we have to adapt to it.
    – Grigory Rechistov
    Aug 20, 2020 at 20:45
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    I hate watching most videos too, but the Park Tool videos are excellent as compared to most random bike repair tutorials on YouTube. They have good production values and often include extra graphics to illustrate complex points. And they do have a lot of articles on their site to go with the videos.
    – Zach Lipton
    Aug 21, 2020 at 6:36
  • @ZachLipton There are few really useful bike repair help channels on Youtube, but Park Tool's is surely one of them. It is really up to the person who explains things. Good and passionate teachers are rare in any area of human knowledge, but they do exist. Once you have found such a teacher, it won't matter in what format (video/text/live etc.) they present their material, as they will strive to make it the best possible, even if that would take them several iterations to get it right.
    – Grigory Rechistov
    Aug 21, 2020 at 7:04
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    @goldilocks You will find written instructions (not video) for many topics here: parktool.com/blog/repair-help
    – Szabolcs
    Aug 21, 2020 at 7:25

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