Leaky Shimano XT 4-Piston Calipers. Any Solutions?

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Leaky Shimano XT 4-Piston Calipers. Any Solutions?
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Posted: Nov 17, 2020 at 15:54 Quote
Does anyone have a solution to stop brand new XT M8120 4-piston calipers from leaking at the pistons?

I built a new bike in June and have been having a lot of problems with my rear brakes. I kept getting bubbles while bleeding and eventually settled for a bit of sponginess. I tried bedding the brakes 4 times to stop squealing (Once right after first assembly and 3 times after cleaning with alcohol and sanding the pads and rotors between rides). All 4 attempts failed and squealing persisted.

After a while my rear brakes started to get spongier so I decided to re-bleed and replace the rotor and pads with new ones at the same time to try and address the squealing. After bleeding I discovered that one of calipers was leaking oil from around one of the pistons and it was pooling at the bottom of the caliper (which likely allowed it to find its way onto the pads and rotor and caused the squealing).

Since this was likely the cause of all my problems I swapped the leaky caliper for a new one under warranty, installed it and again was unable to get all the bubbles out while bleeding. After sealing the system and squeezing the brake a few times I discovered that the brand new caliper also has a leaky piston.

I am beyond frustrated at this point. So far 2 out of 3 new calipers I received have been leaky. Am I doing something wrong? Are there any solutions or do I just have to keep sending calipers back until I receive ones that don't leak? Has anyone else had this same problem?

Posted: Nov 18, 2020 at 22:54 Quote
You're not the problem, the calipers are. Had sticky piston issues on a two year old set of SLX's so I've stripped calipers down, cleaned and rebuilt. Still not solved the issue and nothing else I could do so replaced them in the end.

There are some cheap after market replacement 2 piston caliper kits on Ebay and as far as I know a replacement 4 piston kit doesn't exist.

Wouldn't trust after market stuff in this case anyway and Shimano don't sell their piston and seals as spare parts.

Then again why should you even have to try and fix this obvious issue. Not any other option but to send them back and pray for one that works as it should. I agree a very frustrating situation to be in which doesn't inspire confidence in Shimano brakes.

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