Unanswered Shimano M8120 pistons won't retract after only 15 miles

[Janc]

Brand new 4 pots to replace the 2 pots that were sent off for warranty and the rear brake was dragging after only 15 miles. Following actions taken.
Remove reservoir cap screw, removed pads, checked piston movement, all 4 were out and moved a little to varying degrees. Attempted to pus fronts back in - were very stiff but went in. Attempted to puss rear pair in - won't go in last 1.5mm.
Watched loads of videos on cleaning and lubing pistons. Cleaned with alcohol using bud, string and brush. Lubed with Shimano ping brake fluid. Makes no difference.
Tried allowing pistons to come out as far as possible without popping out - get squirts of fluid from one piston (using block on pair that are all the way in) but now one is 2mm out and the other is mm out. They are solid and whilst will come out, won't go any further back in.
I've been using a plastic tyre lever and toothbrush to push.
Given I can't buy replacement ceramic pistons or seals, and a quick search shows no XT 4 pot rear brakes for sale anywhere - might just be able to get a solitary caliper, have sent warranty claim to CRC who supplied the brake. Is it worth popping out the stuck pistons and will I be able to get them back in again? I guess I'm likely to damage the seals if I try to remove them and clean it all? Assume warranty may be void also if I do this.
Want to try and fix this myself as it's an engine drop and faff to remove the complete rear brake and lever.
What cab be preventing the pistons going all the way in? Air, dirt, damaged seals? FYI bike is Commencal so no LBS option and brake was upgrade bought from CRC.
Thanks in advance.

 

[Shjay]

Sounds like you have done all the correct cleaning, possibly only reason pistons won’t fully retract into the calliper is there is too much oil in the system, will need to undo bleed port 1/4 to 1/2 turn on calliper & push pistons back into calliper. Have an old rag around calliper to soak up oil, I just don’t use shimano brakes anymore now a Magura fanboy

 

[Janc]

Sounds like you have done all the correct cleaning, possibly only reason pistons won’t fully retract into the calliper is there is too much oil in the system, will need to undo bleed port 1/4 to 1/2 turn on calliper & push pistons back into calliper. Have an old rag around calliper to soak up oil, I just don’t use shimano brakes anymore now a Magura fanboy

Thanks for the reply.
I'm working with the lever reservoir open and the brake lever open as well as the brake point screw open. I also tried sucking some fluid out of the system from the caliper nipple but I did that with the syringe rather than gravity. The front pair went back in, it's the rear pair that won't.
Given when I pump the lever to get the pistons out to maximum extension one of the pistons weeps I doubt the system is over full.
What if I drained all the fluid!

 

[Mcharza]

How old is the oil? Maybe you should change oil.

 

[Janc]

How old is the oil? Maybe you should change oil.

What does oil age have to do with being able to move a piston? Fyi brand new latest XT 4 piston complete brake system only done 15 miles.

 

[steve_sordy]

I had a cracked piston on my relatively new Shimano 4 pots. Cracked and jammed in the calliper, was a write off. New calliper under warranty no problem.

 

[Mcharza]

What does oil age have to do with being able to move a piston? Fyi brand new latest XT 4 piston complete brake system only done 15 miles.

Don't be so rude. Of course it has an effect. I just didn’t read carefully enough that they were new brakes

 

[Janc]

I had a cracked piston on my relatively new Shimano 4 pots. Cracked and jammed in the calliper, was a write off. New calliper under warranty no problem.

Hope there's some quick replacements available

 

[Shjay]

Thanks for the reply.
I'm working with the lever reservoir open and the brake lever open as well as the brake point screw open. I also tried sucking some fluid out of the system from the caliper nipple but I did that with the syringe rather than gravity. The front pair went back in, it's the rear pair that won't.
Given when I pump the lever to get the pistons out to maximum extension one of the pistons weeps I doubt the system is over full.
What if I drained all the fluid!

Don’t worry about draining the system, also forget lever it’s calliper bleed port you need undone as push pistons back into calliper but sounds like probably need to send back for warranty

 

[Janc]

Don’t worry about draining the system, also forget lever it’s calliper bleed port you need undone as push pistons back into calliper but sounds like probably need to send back for warranty

Okay thanks I'll try that tomorrow. Still got at least 2 litres of fluid I can pour onto the garage floor....

 

[Janc]

Don't be so rude. Of course it has an effect. I just didn’t read carefully enough that they were new brakes

I wasn't being rude, genuinely asking what dirty fluid would have to do with pushing pistons back. Also just pointing out brakes were new so you had all the information in case you missed it.

 

[Janc]

Don’t worry about draining the system, also forget lever it’s calliper bleed port you need undone as push pistons back into calliper but sounds like probably need to send back for warranty

bled at calliper - in fact emptied all the oil and no difference. They will come out bit won't go in beyond the same point. Awaiting Warranty claim reply from CRC

 

[Jon Sonneborn]

Brand new 4 pots to replace the 2 pots that were sent off for warranty and the rear brake was dragging after only 15 miles. Following actions taken.

I’m having similar issues with my Shimano BR M8120 4-piston rear caliper. On the very first ride on my new bike I was getting a strange moaning sound after releasing the rear brake on high speed downhill sections. This continued on several rides until my LBS bled and cleaned the caliper. Now 6 months later and the caliper is not releasing. I pulled the pads and found the 2 outer pistons were not retracting. I cleaned and worked them back in with a plastic tire lever but am concerned that this is a faulty caliper. LBS is checking with Shimano on Monday to see if they’ll warranty.
My other (4yr old) bike has Shimano saint 4-piston brakes and those have always been perfect!

 

[Doomanic]

I've had to change the rear caliper on my Rail this weekend because the pistons were sticking on one side. They'll extend OK, but won't fully retract, even with a brake piston tool.

 

[Janc]

Shimano warrantied the caliper. Seems the fine grit locally damaged the ceramic pistons!