Leaking shimano XT M8100 rear brake piston?

I assume you've checked the banjo/hose connection, etc for leaks to be sure fluid isn't leaking from somewhere else.

I'd recomend wrapping a paper towel around the bleed block and keep your brake lever pulled with an elastic band trick for a few hours which should let you know which piston the leak could be coming from.

I've never used them myself but there are aftermarket piston and seal kits on ebay you could try?

Alternatively I guess it's gonna be a new caliper or similar second hand caliper you could perhaps scavenge matching piston and/or seals as replacements.

Using a piston and seal from an old caliper I had in my spares bin did the trick and sorted a similar issue for me.

Yes, I did check the banjo bolt. So removing the piston, cleaning and reinstalling wouldn't help? Good idea on the paper towel and bleed block...I'll give that a shot.

I had a leak in Shimano XT 8120 caliper. It wasn't even visible, but the squealing noise kept coming back, no matter what I, or the shop, did. By elimination we excluded all other causes, and eventually the caliper was replaced under warranty. All is quiet since.

So yes, it can sometimes leak, even when new (which means it's a defect, not a degradation from use)

EJ_92606 said:

Yes, I did check the banjo bolt. So removing the piston, cleaning and reinstalling wouldn't help? Good idea on the paper towel and bleed block...I'll give that a shot.

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I had a sticky piston issue that reoccurred after removing and cleaning the piston on a two year old brake. I totally dismantled the caliper, cleaned everything and put back together which didn't help either (in my case) but it could be worth a shot and nothing to lose by trying.

Seeing as there’s no Shimano spare parts catalogue full replacement, warranty?, aftermarket or scavenging seems the only way unfortunately.

I bought a 'fitted but never used' M7100 caliper to replace the old M7000 which also wasn't working right and I couldn't recess outboard piston, turned out the piston was shattered inside the caliper.

I put the 'good' piston from the old caliper in, all sorted and working well for about a year now.

Someone somewhere needs to start selling seals for them. Ridiculous to throw them out for seal. The eBay kits are seal and new piston which is heavier than original piston. Also fairly expensive. We just need a rubber seal.

My preference is to replace shimano calipers with magura when this happens. But then you have to buy the magura hose/banjo kit separately which point the cost is similar to a whole brake (depending on which one you buy) but i prefer the shigura combo. Have them running problem free on two bikes for 1 and 2 years respectively.

I'm willing to bet the piston is broken on the back side.

First way to tell, is if the piston returns flush once pushed back or not. If the piston does not return flush, that's the culprit. The other is remove the caliper by pushing air with a compressor back through the port and pop the pistons out. 9/10 the back of the piston is chipped and not going to seal once moved into operating position. Super common issue I run into regularly, and mostly due to replacing brake pads and going hard on compressing the piston back in. If I see leaking oil from a shimano caliper, I am always going to check for a broken piston.

Note- Shimano does not sell just the piston, you have to either find one yourself and replace, which is easy to do with just your fingers, or buy a whole new caliper.

Did you overheat the brake at some point? I've done this and had similar problems, maybe cooked the seals. Also, there's an o ring between the caliper halves that is notorious for leaking. It can be replaced if you can seperate the caliper. I ended up replacing my rear Zee caliper with a deore 4 piston unit, about half the price and essentially the same thing. $70 now for what was a $40 unit a couple years ago. A lot of profiteering lately.
I've also had the broken piston backside problem that won't allow full retract. How does this happen? You can pop the pistons out with the bleed syringe and a bit of finesse.

Thanks all for the feedback. I don't think overheat is a possibility...have just been doing the normal local rides lately. I also don't think the piston was broken...I used to jam the pistons back pretty aggressively but learned awhile back to put the bleed cup on when changing the pads so they retract quite easily.

I'm going to examine it more closely today to see if I can figure it out. If it is the piston, I'm hoping that I can simply work it back and forth a bunch and that there is dirt in the seal that will come out. Unfortunately my two year warranty expired last month. But if it's the piston leaking I think I will send it into Shimano anyhow and see. I've heard they make exceptions sometimes. But in the meantime I need a caliper as this is my only mtb. Hard to find, but there are a few out there in the US, but about $77, ouch.

To follow up, I inserted the orange plastic piece in the caliper and strapped the lever down tight. Left it that way for a few hours and then looked closely at the caliper from the bottom with a flashlight and no sign of fluid. Took the pads out and saw fluid on the spring and orange piece. So seems that the top of the right side piston is leaking. Found a new caliper on Ebay from a domestic seller and ordered it. None of my local shops or the major online shops had one in stock, so I feel lucky to have found it. Will send the old one into Shimano and hope for a goodwill warranty and then I can have one as a backup. Same thing I did with my broken Shimano shifter....argggh, Shimano come on!

No piston abuse with mine. As I mentioned, when changing pads I put the bucket on the reservoir so the pistons retract easily. 2 year warranty just expired. FYI, for Shimano warranty you don't need to go through a dealer. There is a form on the website. Shimano is not Race Face, they are actually pretty good with the warranty, but not sure if they will make an exception for one month past warranty period.

it maybe good for business, but Shimano not selling spares for their brakes is good enough reason for me to boycott them! Not sustainable at all to replace complete pistons or levers due to leaky seals. Makes me angry!

EJ_92606 said:

No piston abuse with mine. As I mentioned, when changing pads I put the bucket on the reservoir so the pistons retract easily. 2 year warranty just expired. FYI, for Shimano warranty you don't need to go through a dealer. There is a form on the website. Shimano is not Race Face, they are actually pretty good with the warranty, but not sure if they will make an exception for one month past warranty period.

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I found that with some shimano models it does not work, you have to open the bleed nipple to push the pistons in. Even though it's supposed to be self adjusting with open ports when the lever is out.

Those aftermarket piston kits look pretty sketchy to me, unless they come from a trusted source.

Following up on this thread, hoping to not jinx myself, but I have the brake working well now. I ordered a caliper but have not installed it and after working the pistons back and forth a lot and going through the full Shimano bleed method, the brake feels great and has not softened after a ride or two as before and last few rides I have removed the pads and examined things and no signs of a leak at the piston. We'll see. thanks for all the advice.