Bizarre Shimano XT brake problem

LJR69

Active Member
Doing my monthly check and maintenance of the bikes and I noticed my rear pads on my mtb were unevenly worn and actually almost worn out (in just 3 months). My bike has 4-pot Shimano XT (8120) . Upon removing the pads I was unable to push in two of the 4 pistons (the smaller pistons). I therefore removed the caliper and stripped it down. Upon removing the pistons from the caliper I was met with what you see in the images below - i.e. the back of the pistons had crumbled! I mean how the heck does that even happen, and to both pistons almost identically?!

As the unit is covered under warranty I've fired an email off to Tredz and asked for a replacement. For now though I've cleaned everything up, put the pistons back in, lubed them with mineral oil, put new pads in, refitted the caliper and done a full bleed. The brakes feel absolutely awesome. In my mind the fluid will take up the irregular shape of the rear of the pistons and apply the same pressure - not a long term solution but I feel comfortable enough for it to last me this week at least.

If I never post again I miscalculated. No flowers.
 

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Mark pallister

Senior Member
Doing my monthly check and maintenance of the bikes and I noticed my rear pads on my mtb were unevenly worn and actually almost worn out (in just 3 months). My bike has 4-pot Shimano XT (8120) . Upon removing the pads I was unable to push in two of the 4 pistons (the smaller pistons). I therefore removed the caliper and stripped it down. Upon removing the pistons from the caliper I was met with what you see in the images below - i.e. the back of the pistons had crumbled! I mean how the heck does that even happen, and to both pistons almost identically?!

As the unit is covered under warranty I've fired an email off to Tredz and asked for a replacement. For now though I've cleaned everything up, put the pistons back in, lubed them with mineral oil, put new pads in, refitted the caliper and done a full bleed. The brakes feel absolutely awesome. In my mind the fluid will take up the irregular shape of the rear of the pistons and apply the same pressure - not a long term solution but I feel comfortable enough for it to last me this week at least.

If I never post again I miscalculated. No flowers.
It’s not un common ,I had it happen a few years ago
it was out of warranty so bought new pistons and replaced them
don’t think you can buy them anymore, the pistons that is
 
OP
LJR69

LJR69

Active Member
Looking on Shimano parts site it seems they no longer have dealer part numbers for the pistons and so you can only get them as part of the calliper.
 

Drago

Flouncing Nobber
Location
Poshshire
It doesn't happen very often at all so that's a silly comment...:rolleyes:
Its a very relevant comment. Such systems aren't faultless, and as they age they are more prone to such failutes. I know this first hand - I have suffered a Shimano master cyclinder failure that required replacing anew, so your assertion is of little comfort.

Discs are superb, but when they do go wrong I suddenly find myself wishing for a device with a cable that can be diagnosed and repaired in minutes, and thats the be bit people forget.
 
For simplicity, reliability and maintenance I think it's hard to beat a good old set of v-brakes. But I think generally disc brakes are reliable enough. I've had early Formula and then BB7 Avid disc brakes with no real issues (actually out mtbing early in a New England winter I had an alarming issue of water in the cables freezing up and well, the brakes couldn't activate no matter how hard I pulled -but wouldn't I have had the same issue with v-brakes anyway?). I've also had a couple of pairs of Avid Elixirs with great success, with no real issues.

The point is that I think as a rule of thumb, disc brakes now are reliable enough and while prone to more failures than say, v-brakes, not enough to be an issue or a reason to ditch them. I always think disc brakes are like derailleurs -I mean derailleurs are more complex than single speeds and probably have more issues, but I don't think people who want multiple speeds will be going back anytime soon.

I just think the advantages of a decent hydraulic disc brake far outweigh the negatives now.

Its a very relevant comment. Such systems aren't faultless, and as they age they are more prone to such failutes. I know this first hand - I have suffered a Shimano master cyclinder failure that required replacing anew, so your assertion is of little comfort.

Discs are superb, but when they do go wrong I suddenly find myself wishing for a device with a cable that can be diagnosed and repaired in minutes, and thats the be bit people forget.
 

ChrisEyles

Veteran
Location
Devon
Its a very relevant comment. Such systems aren't faultless, and as they age they are more prone to such failutes. I know this first hand - I have suffered a Shimano master cyclinder failure that required replacing anew, so your assertion is of little comfort.

Discs are superb, but when they do go wrong I suddenly find myself wishing for a device with a cable that can be diagnosed and repaired in minutes, and thats the be bit people forget.
I've had sticky pistons on my calipers which required a lot more faff to fix than nipping up a cable. Brake bleeding I also find a bit of a PITA. It's not the end of the world if I can't ride my MTB (it's a toy, not a commuting tool!) so I'm good living with the (low) risk of failure for the nice lever feel and self adjusting nature as the pads wear.

However I recently stuck some mechanical Avid BB7 calipers on a drop bar build (hydraulic STIs were way too expensive!) and have been very impressed with them. If I was relying on a bike for commuting, I'd probably rather it had cable discs than hydraulic, just for peace of mind.

For me, the slight increased risk of pesky mechanical failures of a mechanical disc caliper over vee/canti brakes is totally worth it for the increased performance off-road in the mud, or even on-road on wet and filthy back lanes.
 

Drago

Flouncing Nobber
Location
Poshshire
Aye, some superb cable disc systems out there these days. Hydraulics on a bicycle are an answer to a question no one was asking, and it pains me that so many of my bikes have them because when they fail - and one day they will, and one has already - the likelihood of being able to effect a repair instead of replacing components wholesale is very low.
 
OP
LJR69

LJR69

Active Member
I love 'em. I mean for mountain biking I honestly believe they come into their own. I could go either way with the road bike, but mine just happens to have them.
 
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