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Discussion Starter · #1 ·
Some time ago I noticed my XTR BL-M9100 master cylinder was leaking from under the black plastic end cap. Hit up my LBS and I got a replacement master cylinder / lever as some r*tard at Shimano deemed it best not to sell repair kits or spares for these brakes!

I install and bleed the new master cylinder / lever and everything is peachy until the third ride in I hear a POP mid ride and again have a leak from the black plastic end cap! The brake still works as long as I keep my bike upright, but I'm sure this isn't right.

Last night I was changing brake pads and made sure to prevent excess pressure in the master cylinder when pushing the pistons in by opening the bleed screw at the lever. Well, gently pushing the pads in and AGAIN POP! I now have 2 leaky master cylinders of which the other ones has maybe 300km of riding on it! The Shimano guide doesn't mention any special procedures for changing the pads: https://si.shimano.com/pdfs/dm/DM-MADBR01-00-ENG.pdf

Am I alone with these problems? I find it ridiculous that you can't rebuild these brakes and the only option is to replace the entire master cylinder/ lever.
 

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You mean leaking where the hose exits the master cylinder? Check page 9, you see the parts there:

cut the brake hose so that the cut end is perpendicular to the length of the hose. If the brake hose is cut at an angle, fluid leaks may result
check that, it's possible to get replacements (cheap) and a tool to cut the pipe (SRAM has one which is not to expensive). So you can redo that, and install to the right torque. See pages 26-33. It's not hard to do, if you have the tools.
 

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That is a very useful post.

I'll disassemble and have a look inside. Wonder if the diaphragm aspect of the design actually needs to be a thin flexible material, or something more sturdy solid plastic can be glued in place.

Any thoughts on the diaphragm and what purpose it serves would be most welcome.
 

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Discussion Starter · #8 ·
The diaphragm is very thin and shaped so that it guides oil to the port leading to the master cylinder. It doesn’t actually reach the port so perhaps someone more well versed in the design can comment.

sealing the end cap without the diaphragm would be rather difficult though. So from that POV it can’t be eliminated completely.
 

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Unlike cars and motorcycles that have an open resivour, our brakes have sealed hydraulic system, so yeah it needs the flexible diaphram.
Ive never had probs popping any of mine, but i always fully open and remove the resivour plug and install the bleed cup when servicing brakes. That way there is minimal pressure in the resivour when pushing fluid from the caliper
 

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Discussion Starter · #13 ·
What distracts me is the shape of the diaphragm. I’m aware it’s used to separate air and fluid, but why the strange funnel shape? I assumed it will “float” on the fluid, but upon further inspection it appears to sit at the bottom of the reservoir instead.
 
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