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I have Formula T1S disc brakes (2014 or 2013 model). One day when it was quite hot (in the 30s), but otherwise on a very mild ride on bike paths with kids, the brake started rubbing constantly by itself, grabbing the wheel quite strongly as if on both sides, giving me significant resistance (couldn't even spin the wheel freely). I couldn't reset it using normal disc brake alignment procedure (loosen bolts, hold lever, tighen while holding).

Upon more careful examination, when I removed the brake pads at home, it turned out the pistons inside are pushed inwards, not flush with the brake body, and I can't push them back outwards - I mean both at the same time: when I push one outwards, the other goes in.

How to diagnose how to proceed from there? I haven't serviced these brakes since I bought the bike 5 years ago and in fact it was my first bike with disc brakes, I don't have a lot of experience here. Other answers and various videos suggest a full bleed procedure which means I should probably bring the bike in to the LBS, as I don't fancy buying a full bleed kit to do something once every 5 years... and I don't even know if it would solve the problem.

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Seeing the pistons extended a little is expected on hydraulic disc brake calipers. As the pads wear fluid is slowly drawn from the reservoir and the pistons extend to take up the space left by the pad wear. The pistons should retract slightly when the lever is released to back the pads off the rotor though. It sounds like that failed to happen with your calipers.

You should be able to push both pistons back. This can be be tricky as you have to put opposing force on both pistons. I have a set of tire levers that work well for this, but y9u may have to search for the right tool.

Dirty pistons and seals may be the cause of the pistons not retracting properly. You can try cleaning them and see if that helps. Park tool has a good video showing how that is sone, and it also shows how the automatic pad wear adjustment works.

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  • Thank you, that looks like something I might be able to try! But as I understand, first I have to buy a little DOT4 fluid for cleaning. Two more questions after watching the video: 1) Isn't this stuff corrosive? Can I clean the pistons with something else? 2) How do I know how far it is safe/necessary to push the pistons out?
    – ttarchala
    Nov 29, 2020 at 16:39
  • @ttarchala ~3mm is safe.
    – MaplePanda
    Nov 29, 2020 at 17:15
  • In the end I managed to obtain a little DOT4 fluid as well as the special Park Tool spreader "shovel". I pushed the pistons out, cleaned the edges with the DOT4... and that's as far a I could get. I was not able to push the pistons back in far enough -- and believe me, I tried very hard and had good leverage. I barely managed to squeeze the disc back in between the brake pads after the whole operation. While previously it was just rubbing a little, now it's rubbing constantly and stopping the wheel after a half-turn. I guess this means that the full bleed operation is now inevitable :-(
    – ttarchala
    Dec 26, 2020 at 16:07

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