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I just bought a new road bike with hydraulic disc brakes, using a Shimano Ultegra R8020 Groupset. The pad was rubbing on the rear rotor and it wouldn't recenter when pumping the brake. The brake lever was also very tight at the time (much more than the front lever).

I took the wheel off and tried to press the piston in but I must have pressed too hard because it went in all of a sudden and fluid squirted out of my break lever (under the hood). The brake lever is now very loose and has about 5% braking power.

I assume I broke some seal or something... Any ideas on WTF I just did?

Info from repair:

The LBS said I pushed out a valve or plug - They said on some MTB systems there is a one-way valve that will deal with this, in case there is too much pressure, but the system I have lacks it. It was a simple fix, they pulled the plug, cleaned it, bled the line and replace the plug.

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    This is the kind of question where the idea of asking or answering in a way that's somehow universal to all road hydro brakes just isn't very helpful, so please do let us know what brakes you have. That said there's basically three possibilities here: a seal failed or wasn't present, the hose connection or the hose itself failed, or the bleed screw was loose. Whatever happened, sounds like you did nothing wrong; there's not really supposed to be a "too hard" when pressing the pistons back (within reason). Everything holding the fluid in has to be able to withstand way more pressure than that.
    – Nathan Knutson
    Commented Jun 19, 2018 at 21:13
  • @Adam can you roll back hood section near the clamp and tell where the fluid came from? There's a bleed port in the top mid section and a hose connection near the clamp.
    – Klaster_1 Нет войне
    Commented Jun 20, 2018 at 1:30
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    Consider yourself lucky this happened while doing maintenance, rather than during a hard braking incident on the road.
    – Criggie ♦
    Commented Jun 20, 2018 at 1:32
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    @Klaster_1 - I just took it into the shop to have it looked at. The port seems fine, and when you pull the lever, you can see small bubbling near the port, but not at it. The mechanic said he has never seen anything like it before and will need to take it apart to see what happened
    – Adam Meyer
    Commented Jun 20, 2018 at 1:52
  • @AdamMeyer I'd post the second update as an answer and accept it.
    – Klaster_1 Нет войне
    Commented Jun 24, 2018 at 7:35

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The LBS said I pushed out a valve or plug - They said on some MB systems there is a 1-way valve that will deal with this in case there is too much pressure but the system I have lacks it. It was a simple fix, they pulled the plug, cleaned it, bled the line and replace the plug/valve. - They charged $40 (in a super expensive area)

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