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I have a bike with a hydraulic disk brake and have been observing some degraded performance recently.

Yesterday I finally had a close look and noticed that one brake pad permanently touches the rotor. I tried fixing this by loosening the caliper and re-tightening the screws with the brake lever pulled, but this did not solve the problem.

So I completely removed the caliper and noticed that one piston is almost completely depressed, while the other is protruding a lot. I used a flat tool to push back the protruded piston and reassembled the whole system, but now my braking power is almost completely gone. No matter how often I pump the lever, the brake will barely stop a free spinning wheel.

What has happened? Did I suddenly get air into my system by pressing back the piston? Do I need to bleed the system to get back my braking power, or is there a simpler way? I do not feel like buying the equipment required for bleeding the system at this time.

If it matters, my bike has a Shimano BL-MT200 brake lever and a BR-MT200 caliper.

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Does the brake lever feel "soft" compared to the other brake lever? If so, that's an indicator of air in the line and a bleed would be the answer.

If your pistons are gummy and dirty it would pay to clean everything, and potentially flush all the brake fluid then replace it all.

If you feel uncomfortable working on the safety-critical brakes, there's no shame in getting a shop to do it for you.

These are entry-level hydraulic brakes - if you have any doubts then a complete replacement is also relatively affordable.

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  • I have one personally, they're fine brakes so don't let "entry-level" be a turn-off.
    – Criggie
    Jul 15, 2022 at 6:53
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    Yes, it feels extremely soft. Can pull the lever all the way to the handlebar without much resistance. So it feels like lots of air in the system, but I am wondering how this could happen from only pushing back the pistons. Would be a waste if I bought the tools for bleeding only to find out the issue was something else. Though I cannot think of any other explanation, hence I ask.
    – laolux
    Jul 15, 2022 at 8:32
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    @laolux the tools aren't too expensive, and if you need them once you'll likely need them again in the future. You don't need bike-shop grade fancy ones, just plastic is ample. Store them in a clean empty icecream container together, for next time.
    – Criggie
    Jul 15, 2022 at 8:53
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    usually I would totally agree with you, but I currently have very limited space to store things and will soon have to move long distance, making it difficult to transport many belongings. And I dread the task of finding out the local regulations for disposal of old braking fluid.
    – laolux
    Jul 15, 2022 at 12:30
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    @laolux then pay a bike shop to do the bleed for you, probably the best+easiest solution.
    – Criggie
    Jul 16, 2022 at 0:43

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