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Discussion Starter · #1 · (Edited)
TLDR at the bottom if you don't want to read all this ;)

I acquired a full set of Shimano XT 4 pots (M8120s) a few months ago, replacing the previous MT-501s I had on.

When I bought them, I cut the rear line a little too short, meaning if I turned right too much, brakes go bye bye. I mounted it to the top tube with zip ties and forgot about all about it, telling myself I'll get a new hose whenever.

Months later, I finally decide to stop procrastinating and replace the hose. I bled both my brakes, replacing my rear brake line. I also bled em with the free stroke out, and now it actually works! (at least... just on the front for now)

Now comes to the issue: I noticed my rear brake was rubbing, my pistons were slightly distended so I grab a pad spreader, push the brake pads apart, only for the valve's seal (as pictured) on my rear brake lever to give way and shoot fluid on my shoulder.

Naturally the lever now goes almost all the way to the bar now. I tried a quick burp to give it the necessary fluid, and it didn't really do much, other than the valve that had previously taken fire on me leaking a bit.
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Were my brakes overfilled? What's that valve even for? I've noticed in the past the valves seemed to leak just a bit if I'd burp the lever, but paid no real attention to it...

TLDR
Pushed my brake pads back, because the pistons were out, and my lever shot fluid on me and went flat.
 

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shimano bleed procedures should be enough and not require opening bleed port to push pistons back, so I guess it was overfilled somehow. manual says nothing about opening bleed port to push back pistons if they are contacting rotor

they do mention if you grab the brakes without wheel installed, and pistons move too far out, -and- are difficult to move back, to open bleed port...but elsewhere in the manual, keep it closed

• If the brake lever is depressed without the pad spacers installed, the pistons will protrude further than is normal. Use a flat-shaped tool to push back the brake pads, while being careful not to damage the surfaces of the brake pads. (If the brake pads are not installed, use a flat-shaped tool to push the pistons straight back in, while being careful not to damage them.) If it is difficult to push the brake pads or pistons back, remove the bleed screws and then try again. (Note that some oil may overflow from the reservoir tank at this time.)
 

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That hole is not a valve. Its a breather hole for the bladder. If oil is coming out from that hole, the bladder has popped. You can take it a part and see how big the hole is torn in the bladder. If its small enough, you can clean the bladder of any oil and use super glue.
 

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Discussion Starter · #6 ·
Sounds like the system was filled to the max and the bleeder wasn't opened when you pushed the pistons in resulting in your experience. The fluid, which was being compressed when you pushed the pistons in, simply let go at the weakest point since the system was closed.
Yea, I believe this was the problem. I'll bleed them again, being more careful this time. Thank you all for your help!
 

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The one1 and Tommy E are onto it.

A year ago before I was learning brake maintenance better. I had a shop permanently ruin my front 8120 XT lever due to this issue, I needed a new lever under warranty and learn not use that shop. To reset the pads the upper lever bleed port should be opened first, or the fluid drained otherwise pressure builds too much in lever and it begins leaking.

Check to be sure the lever doesn't leak oil permanently now. Shimano brakes are fairly simple, but quirky, and not forgiving to certain incorrect approaches (myself included).
 
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