Fixing Shimano XT/XTR M8100/M9100 leak from master cylinder end cap

EDIT 29.4.2023: Thanks to Oliver for pointing out that Shimano have now made spare diaphragms and instructions on how to replace them available to the public! https://si.shimano.com/en/dm/MADBR01/replace_diaphram

This is a description of how I went about repairing leaking M9100 brake lever. Only attempt at your own risk!

I’m running XTR M9100 brakes on my Mondraker F-Podium DC and they come with the BL-M9100 master cylinders / levers. I recently discovered a leak on my rear brake lever where fluid was seeping out of the master cylinder’s black plastic end cap. I got a new lever under warranty, installed and bled it and all was peachy until a few rides in “POP” and the end cap was leaking again! The brake still works as long as the bike is kept right side up so I’ve kept riding.

The other night I went to replace the brake pads on my front brake and even took precaution to open the bleed screw on the master cylinder when pushing the caliper pistons in. Mind you, this isn’t something Shimano even mention in their instructions, but low and behold, “POP”! And now both of my master cylinders were leaking from the end cap!

Shimano, in their infinite marketing wisdom, have decided not to make spares available for these brakes so you’re stuck with replacing the entire lever at around 100€ each! Even dealers don't have access to spares, so this effectively makes Shimano brake levers and calipers one-time-use items.

This is a common problem in the following the new Shimano i-Spec EV levers:

BL-M7100 SLX

BL-M8100 XT

BL-M9100 / BL-M9120 XTR

Preventative measures

There are 2 ways to prevent the diaphragm from potentially rupturing. These may or may not work. They didn't for me.

1.) Only bleed the brakes using the Shimano bleed block to prevent over filling

2.) Open the lever bleed screw when changing the pads and pushing the pistons in

The repair

There is a rubber diaphragm under the black plastic end cap, which either seems to deteriorate or is so thin that it ruptures very easily even when pushing pistons in to change pads.

In order to access the diaphragm, you need to open the end cap. It’s held in place by a set screw by the lever. Once the screw is opened the cap can be pulled off to gain access to the diaphragm. In order to pull the end cap off I recommend making an M4 thread on the end cap and using a screw to pull it out. You can pry the cap off, but this will leave marks.

Once you have the diaphragm out, clean it with alcohol and use rubber cement to fix the hole / tear.

The next step is to make an M4 thread on the end cap, if you haven’t already to make pulling it out easier. I threaded a short M4x6 stop screw into the hole and sealed the thread with a sealing compound. This addition is not necessary to fix the leak, as the now glued diaphragm is what keeps the oil in. I decided to add the stop screw for extra security and to keep crud out of the glued diaphragm.

EDIT: I have since learned it’s not a good idea to seal the hole, so only make the thread to help remove the end cap!

Conclusion

This is a modification / repair to do at your own risk! The ruptured diaphragm is a VERY common problem with Shimano brakes and perhaps it's necessary to raise discussion on the futility of offering one-time use parts in the modern world where sustainability is a bigger and bigger concern for many companies. To me this shows complete disregard of the environment by Shimano and I hope they will reconsider their policy on offering brake spares.