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Discussion Starter · #1 ·
I reset the pistons in my XT M8000s to put in some new pads, and the brakes must have been overfilled with fluid. Some fluid ended up leaking out from the plug piece on the outboard end of the master cylinder. The leak happened in one short squirt of fluid, with no ongoing leakage.

Did I blow a seal, or are the brakes designed to bleed off excess fluid? I have ridden the brake since and it feels fine, and I haven't seen any further leaking.
 

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To your question, doubt you've damaged anything, not 100% they have a blow off, but I've done it before and never had a problem after.

Actually what happened is that you had air in the system as your pads wore, then when you tried to push the pistons back in to compensate for the new, thicker pads, it pushed out the fluid that was ontop of the air, you should do a lever bleed at least on your brakes.
 

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I reset the pistons in my XT M8000s to put in some new pads, and the brakes must have been overfilled with fluid. Some fluid ended up leaking out from the plug piece on the outboard end of the master cylinder. The leak happened in one short squirt of fluid, with no ongoing leakage.
That's weird. I've never seen leakage. Is it possible the bleed port plug needs to be tightened?
 

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Discussion Starter · #6 ·
That's weird. I've never seen leakage. Is it possible the bleed port plug needs to be tightened?
It didn't come out of the bleed port. It came out of the outboard end of the master cylinder.

I ask this because I noticed a tiny port on the MC on my Guides that wept fluid when I was bleeding them one time (the SRAM two-syringe bleed procedure makes it possible to over-pressurize the system). So I assume Shimano could have something similar, though the open-funnel bleed procedure makes it more difficult to over-pressurize.
 

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In my case I didn't even notice where oil leaked from. I got desperate that a seal had blown (just like you), but when I put it all together again, all worked fine. But definitely did not leak from the plug.
 

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It didn't come out of the bleed port. It came out of the outboard end of the master cylinder.

I ask this because I noticed a tiny port on the MC on my Guides that wept fluid when I was bleeding them one time (the SRAM two-syringe bleed procedure makes it possible to over-pressurize the system). So I assume Shimano could have something similar, though the open-funnel bleed procedure makes it more difficult to over-pressurize.
Oh. There is a seal that the piston is in that flexes. It's easier to see in this video:
I suppose it's possible some fluid made it past that piston/seal interface. It may be that it won't happen again, but if the seal is damaged, it may continue to leak. That usually results in the fluid occasionally contaminating the pads/rotors. If that happens, you may have to rebuild them or replace them, but it may have just been a one time occurance.
 

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Did you actually lose braking power or did lever travel increase?

I kind of did the same thing with Tektro Draco brakes on the front. I pushed the pads back after installing new rotor. I had to push hard to move them and then some fluid came out of the master cylinder (it is a 2 piece and it came out of where bottom and top connect). My best guess on why is that I bled the brakes with worn pads or rotors. And that was too much fluid for new rotors.

I bled the brakes and got quite some air out. I still have more travel and lack of braking power.

On the rear brakes I was smarter and put the bleed funnel on before pushing the pads back. Now my rear 180mm brake is much stronger than my front 203mm. Lol. So I also wonder if I broke something....
 

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Discussion Starter · #10 ·
Did you actually lose braking power or did lever travel increase?

I kind of did the same thing with Tektro Draco brakes on the front. I pushed the pads back after installing new rotor. I had to push hard to move them and then some fluid came out of the master cylinder (it is a 2 piece and it came out of where bottom and top connect). My best guess on why is that I bled the brakes with worn pads or rotors. And that was too much fluid for new rotors.

I bled the brakes and got quite some air out. I still have more travel and lack of braking power.

On the rear brakes I was smarter and put the bleed funnel on before pushing the pads back. Now my rear 180mm brake is much stronger than my front 203mm. Lol. So I also wonder if I broke something....
No, the power feels fine. It sounds like we did similar things; I had bled the lever partway through the life of the pads, so the brakes were overfilled. I bled my lever after the fluid leak and a big air bubble came out, but the throw feels similar to the rear.

Do you think you maybe contaminated your pads during the leak?
 

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No, the power feels fine. It sounds like we did similar things; I had bled the lever partway through the life of the pads, so the brakes were overfilled. I bled my lever after the fluid leak and a big air bubble came out, but the throw feels similar to the rear.

Do you think you maybe contaminated your pads during the leak?
Contamination is a possibility in case there also was a less obvious leak at the caliper. But I now have lot of lever travel, which couldn't be explained by contamination. I can pull the left lever almost all the way to the handlebar, about twice as much as I can pull the right lever. I can push (not tide) the bike and the front wheel doesn't even lock. I haven't bedded them in yet , but that shouldn't matter that much. I had 180 rotors on and they stopped on a dime, now after upgrading to 203mm they still should stop me bedded in or not.

For the future I won't bleed brakes unless I install new pads. And when I push pistons back I put the bleed funnel on.

I wonder if the Tektro Dracos have less buffer volume to deal with fluctuating fluid volumes and pad and rotor wear.

Btw, old rotor was only 0.1 mm thinner than new rotor. If that.
 

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^^^ remove existing pads, reset pistons, install bleed block, bleed brake, install existing pads. If you suspect excess fluid in the system, crack the bleed screw on the lever to let it out when resetting the pistons. A little fluid and/or air may come out which you can catch with a rag.

I always remove the pads when bleeding to minimize the possibility of contaminating them.
 

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amusingly i just discovered the blow-off / relieve port or whatever on my SLX brakes last weekend.

when to change pads, it wasn't me that bled brakes when bike was set up.. must have been overfilled by tech... anyways fluid blew out end of reservoir by lever when trying to push calipers out to install new pads..

Bicycle part Bicycle accessory Bicycle Bicycle tire Bicycle wheel rim


removed plug and put the little bleed kit cup on and pushed calipers the rest of the way out and remaining excess oil went into the cup...

blah blah ...brakes are fine since..
 

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^^^ remove existing pads, reset pistons, install bleed block, bleed brake, install existing pads. If you suspect excess fluid in the system, crack the bleed screw on the lever to let it out when resetting the pistons. A little fluid and/or air may come out which you can catch with a rag.

I always remove the pads when bleeding to minimize the possibility of contaminating them.
I attempted to this variation that recommends to fill some extra at the caliper. But I couldn't open that bleedport and stripped that 7mm screw. So that didn't work. But luck had it that I had a set of Deore brakes I planned to use on another bike and installed those.
 

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Welp same thing happened to me. Fresh and bnew m8020, installed then bled. Felt fine ehen bedding the brakes but after 15min fluid started to leak at the side. Will re bleed on my day off to see if there is any difference.


Sent from my SM-G965U using Tapatalk
 

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Asked my lbs main mech about this and this happens when people do bleeds with not fresh pads, then pry the pistons back and the fluid needs to go somewhere... and out that port it goes.

He cracks the fill screw when installing new pads generally.

I had some bad bleeding issues and i siliconed that valve shut. Then found it was leaking around the valve, there is a set screw from inside the bar hole that holds it in, i then siliconed the plug in and boom... no more leak

Ended up finding a xt lever for $40 so i replaced it anyways.

Also if your brakes are not as suicidal, back the free stroke screw right out, bleed, or ghetto bleed then when the plug is back in tighten the free stroke and boom... like new!

Want more modulation and spongy sram feeling brakes ? Dont touch the free stroke screw when bleeding.


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safety valve in Shimano brakes? don't be silly. behind the plug with a hole is a thin rubber compensation chamber. if you were lucky, chamber simply jumped off the plug. or rubber membrane was torn and all you can do is replace whole lever body because Shimano does not sell them as replacement part. "user must pay" is an industry slogan )
 

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safety valve in Shimano brakes? don't be silly. behind the plug with a hole is a thin rubber compensation chamber. if you were lucky, chamber simply jumped off the plug. or rubber membrane was torn and all you can do is replace whole lever body because Shimano does not sell them as replacement part. "user must pay" is an industry slogan )
How do I remove that plug?
Как убрать заглушку?) Съёмник нужен?) Или винт вкрутить и за него потянуть?
 

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No. There's a relief valve.
Yes there is a relief valve BUT once it pops, you've pretty much ruptured the bladder. I've accidentally overfilled my brakes in the past and learned the hard way.

You'll have to fully disassemble the master cylinder to be able to remove the bladder. Then fully degrease it and add a dab of Gorilla glue to it. It should last a while. I FINALLY found one store that sells replacement bladders but the basterds charge $30CDN for it. Still cheaper than tossing a master cylinder.
 
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