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My 2012 Model Scott Sub 10 has recently been experiencing issues with the rear disc sticking on my daily commute and getting so bad that I have to stop, wait for it to cool and then push the disc a bit to give it some space. I believe this has been caused by the terrible weather that the UK has experiencing, perhaps with some debris becoming lodged in one of the pistons.

I decided to take the pads out this evening following the guide that came with the bike and cleaned any dirt around the pads but there is still some friction there.

I am debating pouring warm/hot water over the whole brake system in an attempt to remove anything that may still be lodged.

What advice would you you suggest aside taking it to my local bike shop?

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The advice I have seen for a stuck piston is a) hot soapy water, followed by Isopropyl Alcohol, followed by a small amount of brake fluid (Use the same that is in you brakes - mineral in your case) to lubricate the piston. If you google for "Stuck Bicycle Disc Brake piston" there is quite a lot of useful advise

Warm soapy water on it's own cannot hurt. Although I have never needed to do this, I would use a syringe to direct the water at the pistons to waste away as much crud as possible. Move pistons out (being extremely careful not to drive them out too far - put a pair of old pads in before squeezing the lever... When pushing them back in, do it gently) Watch for a piston moving more than the other. If one is not moving, hold the other in to get it free. Then squirt water over them and wipe them off with a clean rag.

If the crud contains too much in the way of oils, the water may not be enough - resort to the IsoP.

Wipe a clean rag soaked in a small amount of brake fluid over the pistons. If desperate, a normal mineral oil, but what every you do, don't use too much. You need just enough to put a thin layer over the piston, and more and you risk getting it on the pads (very bad for them).

Finally, and most importantly, get a clean dry rag and wipe the whole assembly down as clean and shiny as you can. Install pads and see how you got on.

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    Its easier to pop out a piston than you'd think, so be sper careful if you try that. With Shimano brakes there's a good chance its totally knackered if you do that.
    – alex
    Commented Feb 14, 2013 at 8:45
  • Opted for the bike shop option - I haven't had it serviced in 6 months of continued use so it was probably for the best. Cheers for the advice though!
    – Malachi
    Commented Feb 15, 2013 at 11:58
  • @mattnz what about GT-85? Would that work on a stuck piston?
    – Jono
    Commented Jun 4, 2013 at 23:23
  • Ah. Google seems to suggest this would eat through the hydraulic seals. Not good.
    – Jono
    Commented Jun 4, 2013 at 23:30
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I read your last sentence, ok?

But this is not you bell being loose, but something that is critical to your safety. since you are asking these questions, my recommendation is take your bike to a qualified shop and have it fixed propery by some who knows exactly what they are doing. You probably have some road salt induced corrosion on the piston and that can lead to a leaking caliper.

I just finished reading the thread on how to stop your bike if you loose your brakes. You sir, are halfway there. No offense intended here. Joe

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    It's currently getting serviced at the moment at a trusted bike shop. Should be ready to pick up tomorrow unless they need to replace the calipers/disc. Thanks for your advice.
    – Malachi
    Commented Feb 15, 2013 at 11:57
  • A bit OTT playing the safety card on this one. By your reasoning only a "qualified" bike mechanic should perform maintenance on brakes, steering, cranks... A sticking rear disc is incredibly unlikely to suddenly fail - a leaky caliper leads to degradation of performance over time. Even if he has a sudden failure, the front brake (Which according to Sheldon Brown accounts for 100% braking when max stopping is required) is still working.
    – mattnz
    Commented Feb 16, 2013 at 1:02
  • Yes, @mattnz I suppose I am. Working on cable brakes is straight forward compared to hydraulic discs. The brake is already sticking, it is not inconceivable that the situation might be exacerbated. IF the rear brake locks up unexpectedly, you have a potential skid, especially in bad weather. We are taught to use both front and rear brakes to stop safely. I'm old school, critical work should be inspected by someone who knows what they are doing. Respectfully, Joe.
    – Joe
    Commented Feb 16, 2013 at 3:08
  • Got the bike back and the problem was due to one of the pistons getting locked. They replaced callipers for £20. I've only had the bike six months and it feels like this is something that shouldn't fail within this period.
    – Malachi
    Commented Feb 18, 2013 at 15:21
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Hydraulic brakes are easy if you know what you are doing, if you dont then leave em alone. The pistons come out and the seals have to be replaced right way round if they go in the wrong way you have no brakes. Most pros I reckon replace calipers because its easier than stripping down old ones and rebuilding. Also it doesnt take a trained mechanic to do this just the apprentice or whatever.

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