I tried to service the hydraulic disc calipers on my road bike this weekend, found that the pistons wouldn’t push back fully and they were making a horrible crunching sound when I tried to do so! So I used the brake lever to extend and completely remove the piston and I found that the *reverse *face of the ceramic piston (i.e. the side facing the fluid, not the pads) had somehow started to disintegrate, with big chunks of ceramic floating around inside the caliper and finer bits of ceramic dust finding its way all the way around the caliper and back up the hydro line towards the lever. The rubber seal also had some wear, presumably from operating the piston with all that crap floating around inside, but remarkably, once I cleaned it all up, reassembled and bled the system it still works absolutely fine.
Obviously I’m going to replace it ASAP, but I have a few questions, probably for the mountain bikers amongst you who are familiar with using-and-abusing Shimano calipers:
(1) Why on earth would the *reverse *side of the caliper have disintegrated like this and what can I do to stop it from happening again?
(2) Is there a specific reason that Shimano use *ceramic *pistons in their road calipers, rather than metal ones that I’m more familiar with from my mountain bike?
(3) Can I replace the pistons (and seals?) in the caliper, either with fresh ceramic ones or with metal ones?
The crunchy noise was you pushing the pistons off square making them scuff the bore and possibly causing damage.
Shimano ceramic pistons are the best in the business.
They isolate heat from the pads better than any other material.
They also do not corrode if moisture gets past the piston seals.
They maintain their shape better than any other material during high heat stress.
The piston seals were not worn, they are shaped with a square edge and this configuration provides the piston return function when you let go of the lever.
If you have cleaned out any debris, bled the system and there are no leaks under pressure throughout the entire piston travel then you are good to go.
Next time be VERY careful to push the pistons squarely into the bore to replace pads.
I’m fairly certain that the crunching noise was the piston pushing up against all the ceramic piston fragments that were *already *in the caliper body clogging it up! I didn’t try to push them back with much force once it was clear that they were stuck, so I don’t believe that I caused the damage by pushing them back wonky. For example, these are some of the bigger bits that I removed, along with a lot of finer dust and debris that was, by then, spread all throughout the caliper:
The chipping on the back-face of the piston extended a little way up one of the sidewalls, so I’m concerned that as the pads wear and the pistons advance, these chips may push beyond the seal and the whole system will lose pressure, so I’m keen to replace it.
I’m mostly wondering how and why this occurred and what I can do to prevent it happening again. I’m fairly sure it’s not Last time I serviced them the pistons pushed back into the caliper body absolutely fine and now, 6 months later the rear of the piston has disintegrated!
There was a small production run that had problems with cracks in the pistons.
Usually this showed up as slight leaks.
You may find that yours may be part of the problem ones.
See your local LBS.
