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My right brake lever (Shimano BL-M9020 to be specific) has attracted some dirt around the blade pivot area, and some parts look shiny. I suspect the master piston seals are starting to leak. Replacing the entire lever is the guaranteed safe solution, but I cannot afford to do so and I'm unsure about availability as well. Since I enjoy the smooth function, good looks, and low weight of this XTR lever, I decided it is worthwhile to attempt to rebuild it with new seals. How would I do that?

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  • Consider changing the title to Shimano MTB brake levers? I suspect the rebuild process would be similar for most levels of the levers. It could make it easier to find for someone in a similar situation.
    – Weiwen Ng
    Dec 6, 2021 at 11:33
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    @WeiwenNg Unfortunately, this construction with the pressed-in pin is unique to the XTRs. The lower levels of brakes use these plastic guides for the push rod instead of integral metal slots. Guides for rebuilding those are already available online anyways. They’re what I referred to when doing this.
    – MaplePanda
    Dec 6, 2021 at 15:58

1 Answer 1

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This answer must be prefaced with the obvious: brake-related work is high-consequence and could be genuinely injurious if performed incorrectly. Please exercise your own judgement regarding your repair skill level and the risk of the job in question. Read the procedure in full before beginning any work. This answer is merely a guide; in no way am I liable for the outcome of how you use the information presented.


Here are some photos for reference:

enter image description here enter image description here


Tools Needed

  • 2mm hex wrench
  • 2.5mm hex wrench
  • 4mm hex wrench
  • Phillips / JIS screwdriver
  • Flat-head screwdriver
  • Vice grips / locking pliers
  • Assortment of small woodworking nails
  • Needle-nosed pliers
  • Hammer
  • Punch (for use with the hammer)
  • Q-tips / cotton swabs
  • Isopropyl alcohol
  • Paper towel (lint-free is ideal)
  • Low- to medium-strength threadlocker

Some of these items are optional. I sincerely hope I don’t need to tell you when to use what tool if you’re considering performing this repair.


Disassembly

Step 1: On this part of the lever blade, there is a hole. Inside this hole, you will find a rubber plug. Pick the plug out and then loosen the set screw behind that using a 2mm hex wrench.

enter image description here

Step 2: Use a 4mm hex wrench to press the pivot pin out from the bottom (you will remove it from the top side). Be careful not to damage the bushings the pin rotates on. Remove the lever blade, being sure to not lose any of the springs. Remove the freestroke adjustment screw.

enter image description here

Step 3: Using pliers, grab the push rod assembly's roller, push on it to press the master piston in, and with pressure still applied, carefully rotate the push rod assembly clockwise to twist the guides out of their slots. Take care to avoid damaging the blade pivot bushings. Remove the push rod assembly.

enter image description here

Step 4: Clean the inside of the brake lever with the cotton swabs. Avoid excessive use of alcohol at this point to avoid any possibility of contaminating the oil. Remove all dirt and grime so it does not get pushed into the master cylinder.

enter image description here

Step 5: On the backside of the lever body, there will be this small hole. Using a large nail/screw/punch to deliver the first hit and then smaller nails after that, hammer the pin inside this hole out. Make sure you use nails small enough to fit inside the hole as to avoid expanding it. Do not be tempted to press it out from the other end, as the raised ribs on that end of the pin will damage the hole. I improvised a tool by grabbing a large nail with the locking pliers, allowing me to hit the small nail sticking out of the hole.

This is an especially tricky step. You must be slow and methodical, but still hit it hard enough to actually move the pin.

Once the raised ribs have escaped from the lever body, the pin may be pulled out with pliers.

enter image description here enter image description here enter image description here

  • Note here that the deck screw pictured was directly used for only the first hit. For subsequent hits, a small nail skinny enough to fit inside the pin's hole was used, and the deck screw was merely used as an extension to whack on that. The deck screw's tip is evidently far too wide to drive the pin out after the first hit.

Step 6: Press the piston in and simultaneously use pliers or tweezers to remove the black plastic piston retaining ring. This is another finicky step and may require several tries. Afterwards, attach a bleed cup to open the brake reservoir (a pressure differential is generated when the piston is inserted/removed, which could damage the compensation bladder). Situate a catch basin under the bike as oil will flow out. Remove the piston.

enter image description here

Step 7: Install the new seals and/or piston. Halfway done! Take a break now. You don't want to mess up the assembly phase.

enter image description here


Reassembly

This is largely the same as the disassembly, but obviously in reverse.

Step 1: Press in the piston with a hex wrench or something. While keeping it pressed, maneuver the black plastic retainer into position. Make sure to get the orientation correct: the tab should go into the cutout where the pin drives through, and the bevel that the freestroke screw presses on should be facing away from the piston.

As expected, this is a finicky step. It took me a good 15 tries for the first lever and like 10 for the second. It’s really hard to keep the piston pressed in with a tool while still leaving space to insert the retainer. Patience is key; go take a break if you start getting aggravated. Be careful not to scratch the piston bore with your tools while the piston is pressed in.

enter image description here

Step 2: Reinstall the little pin. It’s easier than removal because you don’t need to use tiny nails to drive it beneath the surface (you’re finished once it’s flush), and the front side of the brake lever is much easier to access than the back side. I again used the “deck screw clamped in locking pliers” tool here, but a proper punch would have worked better. The pin will insert smooth end first from the front side of the lever. DO NOT hammer the raised ridges through both sides, you’ll damage the lever body.

enter image description here

Step 3: Reinstall the push rod assembly. Put a dab of grease on the ball-and-socket joint between the push rod and piston. Lubricate the main roller and two guide rollers with either heavy oil or a grease. You will rotate the push rod assembly counter-clockwise during reinstallation, which will let the lower guide slip in first before the upper one does. Again, use your fingers here if possible. I used pliers and unfortunately left marks on the roller’s surface (no gouges or anything thankfully).

enter image description here

Step 4: Reinstall the lever blade. Refer to the image above for the location of the spring’s arm. Grease the push rod assembly slots, the lever blade bushings, the pivot pin, and any other moving interfaces before assembly. The pin will push in from the top. Ensure all parts are aligned correctly before pushing the pin in. It can take some decent force to go all the way through, and you don’t want to break anything. Check that the spring has been correctly installed by pushing the lever blade away from the handlebar. It should return to its resting position under spring tension.

enter image description here

Step 5: Ensure the pin is pushed all the way down, and then fasten the retention set screw. You may want to add threadlocker to the set screw before that. Install the rubber plug in the set screw hole.

Step 6: Reinstall the free stroke screw, bleed the brake, clean up any spilled oil, and give the brake a pressure test by squeezing it really hard. If there are no leaks, you’re done! Nice work.


Notes

  • Again, this is brake work. Your workstation and tools must be clean. Your hands must be free of contaminants or dirt that could make its way into the master cylinder.
  • Remove the brake pads and front wheel before starting work. Oil gushes out when the piston is removed.
  • The procedure is largely the same for the left lever because they are mirror images of each other. Only the directions of movements might change (eg. the push rod assembly is rotated counter-clockwise for disassembly instead of clockwise).
  • Make sure you have plenty of oil stocked for the subsequent brake bleed.

Sourcing new seals

I went on AliExpress to purchase these titanium pistons, which came with new seals. You could replace just the seals on the factory piston, but why not add a little [invisible] bling while I'm in there anyways?

If you don't trust those, you could disassemble a cheaper lever to harvest its piston. They should be the same size for all levers since virtually all Shimano brake components are inter-compatible.

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  • 2
    I like it. Despite the risks involved, it's Q & A like these that are the most interesting and potentially valuable for the mechanic-hobbyist. Thank you.
    – Jeff
    Dec 5, 2021 at 7:08
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    Well done @MaplePanda. (I made an account just to commend you on your thorough self-answer).
    – Stephen
    Dec 6, 2021 at 2:43
  • @Jeff My pleasure — it’s fun exploring the dark side of repairs like this :D I’ve gained a much better understanding of how Shimano brakes work now too, which is great. Worst comes to worst, they start leaking again and I need new levers. I don’t think complete brake failure is a realistic failure mode.
    – MaplePanda
    Dec 6, 2021 at 9:18
  • @Stephen Wow, much appreciated! I’ve struggled through far too many repair guides myself that lack the detail needed to follow through the first time I perform the repair. Hopefully, I did better than that here.
    – MaplePanda
    Dec 6, 2021 at 9:20

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