So.... this happened (ceramic piston breakdown)...

cp_erider

Member
Aug 8, 2022
23
19
Peoria, Az
So I was at Hawes 2 weeks ago (some of the best biking in Az) on Red Mountain Rush .. a fairly long downhill but not something I havent done a zillion times on my "acoustic" bike with XT brakes and sintered pads. I happened to be on my Giant 2022 Giant Trance X E+ Pro 0 (with Live Valve :) and it also has 4 piston XT M8120 brakes (about 8 months old, mod use). The front brake faded towards the bottom and then howled all the way home. The pads had been heavily glazed and rotor was in tough shape. I tried to rehab them but no joy. Fast forward, I put MTX Gold HD ceramics and an new IceTech (203mm) rotor and all good again on RMR ..except I can't get them to stop "singing" because the pistons on the inside will not fully withdraw. The rotor is literally perfect. I just have no play left for the rotor to run with out touching either of the pads. I took the caliper off and tried to force the pistons back in. On the 2 inside cups, they would not budge for anything. With nothing left lose, I popped out one of the 2 stuck piston and find bits and pieces of the back of the ceramic piston preventing it from withdrawing as it should. Obv will be contacting shimano .. but I've been in the business for a lot of years and never seen anything like it. I have an old caliper laying around that I think has compatible pistons .. we'll see. Otherwise I'm gonna mix & max a Saint caliper to my XT lever. I am NOT a downhiller by any means. I do ride hard but had this bike for about 9 months with no problem until that one run. Guessing it happened when thing was screaming .. which is just another way of saying high cycle vibration. Still ... would never have expected this ...
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cp_erider

Member
Aug 8, 2022
23
19
Peoria, Az
What angers me the most about this is that the Chinese can produce a ceramic spare (with dubious credibility) while Shimano tells me to pound sand. The most expensive part of the system is bricked for the cost of a $10 piston. grrr.
 

Mikerb

E*POWAH Elite World Champion
May 16, 2019
6,231
4,724
Weymouth
the seal mark on the piston suggests it was being forced to operate at an angle suggesting the calliper may have been out of line or it moved. The piston is also pretty dirty.......do you clean and lube them occasionally?
 

Dax

E*POWAH Elite World Champion
May 25, 2018
1,532
1,901
FoD
Probably cracked pushing it back in with a tyre lever or screwdriver previously. I’ve seen people take chunks out of them before.
 

Mikerb

E*POWAH Elite World Champion
May 16, 2019
6,231
4,724
Weymouth
If pistons refuse to go back into the calliper with a reasonable push it is best to unscrew the bleed port on the lever. Pushing the pistons back will push a little fluid out which shows that the overall volue of fluid had become bigger due to water contamination or air ingress.
 

cp_erider

Member
Aug 8, 2022
23
19
Peoria, Az
all proper thoughts ... I have a proper tool for pressing pads all the way back and don't use anything directly on the ceramic pistons. This caliper worked perfectly for about 9 months until I burned the pads on a long downhill and they faded. The brake then howled every time I touched it until I got back. I promptly replaced the pads with ceramics and resurfaced the rotor .. but... the pistons on one side would not retract for anything. I did actually open the bleed port and they still would not retract (using the proper tool, not contacting the ceramic directly). It could have been forced to operate at an angle, tho I don't know how since things were fine until that one high speed downhill. Appreciate the thoughts. I'm not going with an amazon piston .. it's half the price of a new caliper. I ordered a new one and wil keep the other 2 pistons for someone else who has the same problem. Oh ... they were dirty, yes. But I do service them .. I think what we are seeing was from that one run / day that was so harsh on the brakes.
 

Chrysaor

Active member
Apr 12, 2021
113
105
Switzerland
They are super fragile I’ve broken a couple of them over the years. The Aliexpress replacement ones work like a charm in case somebody is reluctant
 

cp_erider

Member
Aug 8, 2022
23
19
Peoria, Az
Be careful not to let any ceramic fragment in the circuit before you rebuild it
Good call .. I already discovered fragments in the fluid transfer channels .. I need to take it all apart and use compressed air to clear everything. I actually found fragments from on crushed piston in the chamber behind the piston next to it. Thanks.
 

cappuccino34

Active member
Nov 24, 2020
530
328
Helmshore
I bought some spreaders off the tool van that comes around fortnightly. I don't actually know what their official application is, but they work like a charm for spreading pads squarely. I'm pretty sure they are made by laser, but I'll try and get a part number for them.
 

Another rider

Member
Sep 27, 2020
39
19
Dorset
Same thing happened to me a while back on my Focus. Seems the piston jammed, maybe down to some dirt getting in. I was really surprised you can't buy replacement pistons and it cost me a new calliper. Took the original back to the dealer who said they would have to return it (to Focus) but never got anything back even though it was less than a year old. I wish in retrospect I'd just kept it for spares for next time one breaks.
 

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