Disc brake failure question - Page 2
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  1. #26
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    Never used sharp tools, always used plastic tyre levers for pushing the pistons in.
    I am going to take apart my other set of RS805, and amalgamate all the good pistons to get one set back up and running.
    There is one glaring effect on cracked pistons is that the small shards will make their way to the seals and start to score and cut the rubber seal which would or could be detrimental.
    My say is if your brake pistons are asymmetrical, and pushing back the stuck out piston feels tough and worse crunchy then cracked pistons are the most likely scenario.

  2. #27
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    Quote Originally Posted by Seven1816 View Post
    Never used sharp tools, always used plastic tyre levers for pushing the pistons in.
    I am going to take apart my other set of RS805, and amalgamate all the good pistons to get one set back up and running.
    There is one glaring effect on cracked pistons is that the small shards will make their way to the seals and start to score and cut the rubber seal which would or could be detrimental.
    My say is if your brake pistons are asymmetrical, and pushing back the stuck out piston feels tough and worse crunchy then cracked pistons are the most likely scenario.
    Any chance that the pistons were pushed back too far into the caliper at one time, when you were replacing pads? Otherwise, I can't see how/why the back side of those pistons would crack.
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  3. #28
    'brifter' is a lame word.
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    It would be nearly impossible to push them in 'too far'. I guess you could push 'too hard' and/or use the wrong pusher.
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  4. #29
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    Quote Originally Posted by davesupra View Post
    ...I can't see how/why the back side of those pistons would crack.
    Let me try to explain how/why based on my theory of installation misalignment as the root cause.

    If the caliper is misaligned against the rotor, when the brake is applied, the piston pushes against the pad, but only a portion of the pad would contact the rotor initially, which puts an uneven load on the piston. Since there is a seal between the piston and the caliper cylinder, that seal can act as a fulcrum like a see-saw, pushing the "back" side of the piston wall directly opposite the uneven load against the caliper wall.

    Granted we are talking about minute movements, but I suspect the piston wall is quite thin and not designed to take this type of bending load. Adding to the fact that it is made out of brittle ceramic, it is sufficient to cause the piston wall to fail. One way to confirm this is to examine the caliper body for any unusual wear marks on the cylinder wall.

  5. #30
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    Quote Originally Posted by nova_rider View Post
    Let me try to explain how/why based on my theory of installation misalignment as the root cause.

    If the caliper is misaligned against the rotor, when the brake is applied, the piston pushes against the pad, but only a portion of the pad would contact the rotor initially, which puts an uneven load on the piston. Since there is a seal between the piston and the caliper cylinder, that seal can act as a fulcrum like a see-saw, pushing the "back" side of the piston wall directly opposite the uneven load against the caliper wall.

    Granted we are talking about minute movements, but I suspect the piston wall is quite thin and not designed to take this type of bending load. Adding to the fact that it is made out of brittle ceramic, it is sufficient to cause the piston wall to fail. One way to confirm this is to examine the caliper body for any unusual wear marks on the cylinder wall.
    I started to rebuild the calipers today, so I took out the seals, amazed the inside of the seals had lots of tiny shards of ceramic, feeling the piston area the reservoir behind the seal is actually a bigger diameter than the lip after the seal, so looking closely there is no way the piston can touch and scrape against the side of the fluid filled area. I can only deduce that Shimano are using or used some badly made ceramic pistons.
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