[ccm]

My problem is related to wandering bite point, but a bit different.
After the last bleed my XTR M-9000 rear brake works without wandering bite point like it should while riding, but if I squeeze the brake lever slowly it goes to the bar; whereas, when I squeeze the lever rapidly (just one squeeze & not the repeated squeezing to pump it up) the pads contact the rotor when the lever is half way to the bar (or where ever I adjust it to)
I had the stereotypical wandering bite point and did the brake bleed where you remove the caliper from the bike to hang vertical and removed the bleed screw completely and let the brake oil flow down from the funnel and out the caliper into a rag while rotating and tapping the caliper around in all directions. Finished off by bleeding normally and burping some more oil out after pads stepped and adjusted pad to rotor spacing.
I have used all bleed methods recommended including gravity bleed to hose attached to bottom bleed screw, syringe from the bottom, syringe from top and bottom, tapping, rotating, hanging etc.
When I burp the lever (attach oil filled funnel and squeeze lever to pump air bubbles out and nothing more) when the pads are partially worn, I get oil squirting out the bypass port in the lever and onto my grips, but the brake works after (either correctly or when pumped up for wandering bite point), so after the initial squirt there is not more oil loss.
I am wondering why the difference between slow vs fast squeeze.
I finished my ride about 4 hours ago and just now tested the lever again by first squeezing fast (it worked perfectly), but my second squeeze was slow and the lever went to the bar

Maybe the inside of the master cylinder is worn (almost 5 years old now) or burping the lever so that the bypass port was damaged?
Does anyone know of another reason?

 

[trel]

Likely suspects.

- Torn or displaced bladder/diaphragm inside reservoir. Repair/replace/reseat.

- Damaged or displaced cup seal on master cylinder piston. Replace/reseat.

- Worn master cylinder. Replace full lever assembly.

Can’t get spares new, only parts from a donor lever.

Strip and inspect the lever. Find and remove the grub screw for the end cap. Carefully remove the end cap, it is not threaded, it will pop out if you pry it, but very easy to damage the lever body.

I’ve had a few leak with a tiny hole in the diaphragm, caused by me overfilling the system, or forgetting to remove the lever bleed screw while bleeding.

I’ve had success sealing holes in the diaphragm using automotive engine oil/fuel resistant rtv sealant, from the dry side.

Pictured are bl-m9100 and bl-m8000, your bl-m9000 is likely very similar.

 

[trel]

More detail from @MTB Engineer's blog

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

 

[MTB Engineer]

The bursting diaphragm is a super annoying “feature” with Shimano levers only made worse by the fact they don’t sell spares…

 

[ccm]

Strip and inspect the lever. Find and remove the grub screw for the end cap. Carefully remove the end cap, it is not threaded, it will pop out if you pry it, but very easy to damage the lever body.
Pictured are bl-m9100 and bl-m8000, your bl-m9000 is likely very similar.

My bl-M985 lever has the grub screw, my bl-M9000 doesn't (I even looked in the holes with a 16X magnifying glass, and it looks like there is a pin where the grub screw should be, so maybe Shimano doesn't plan on anyone taking the diaphragm assembly out)
Anyway, I pulled another bl-M9000 from my scrap bin apart for practice. Pulled the piston and spring out of master cylinder and there were some streaks in the bore (scouring?) and the piston was very sticky and I could barely get it to move (which is why it was in scrap bin)

 

[ccm]

From Kenny on nsmb.com
"Sounds like master cylinder is bypassing. Need a new lever. A quick tap and fluid can't bypass the o-rings, but steady pressure it is leaking past."

Looking at the inside of the lever while playing around with the individual pieces, I agree.

My diaphragm has worked as it should. It allowed excess volume to squirt out when I over-filled while burping with worn pads, and never leaked again while in use.


"

 

[MTB Engineer]

The diaphragm is not designed to burst or leak. It’s a complete oversight on Shimano’s part and an easy part to break and replace IF spares were available.

Besides, unless you bleed your brakes with worn pads and not using the Shimano bleed block there won’t be excess fluid in the system that needs “burping”.