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I have Shimano M785 hydraulic disc brakes.

It is easy to remove the pads and put them back in again even with the disc in place. However, resetting the brake pistons is tricky. I currently use a butter knife and two hands, which mostly works but is very awkward.

All the directions I've seen for resetting pistons (for example) call for removing the wheel. I'd would like to avoid this step if it's not essential; it transforms pad replacement or access to the pistons from a quick 5-minute job to one that requires a workstand and may be dirty due to chain contact. For example, this is relevant when diagnosing issues that require resetting the pads more than once, or perhaps in a shop where time is money.

Is there a tool or technique that will let me do it easily without removing the wheel?

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    Why would you not want to remove the wheel? It's not like you're going to have to do this every day. – Batman Aug 23 '15 at 1:48
  • Why should you reset your pistons? Be careful: if you push brake lever while there is no pads, you can cause air inside the system, which require to bleed it. – Alexander Aug 23 '15 at 12:33
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    @Batman: Because it's a pain, particularly for the rear wheel. Also, many repair jobs done by inexperienced people (i.e., me) require iteration or several tries before it's done right. Why do unnecessary work? If you believe the answer is "it can't be done", please write an answer to that effect. – Reid Aug 23 '15 at 14:27
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I believe it is not worth trying it. You risk contaminating, scratching, or bending your rotors, as well as scratching or cracking a brake pad.

Consider also whether there are bigger problems with your bike setup if it's too much hassle to remove your rear wheel to change your brake pads — how often does that happen: every 2–3 thousand kilometres? Is there something complicating the process that you've not told us? Is this perhaps an electric motorbike? Or is it a belt drive?

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    Thank you for taking the time to answer. However, I must downvote: (1) Risks of error are vague. What, specifically, is the the problem here? You can assume that readers know not to lever off the disc directly, are aware of contamination issues, etc. (2) Value judgement. This would literally take a job requiring a workstand and handling greasy chains to 5 minutes in and out with the pads. You may not value that, but that doesn't mean others don't. Please accept the premise of the question. What I'm hearing with this answer is "I don't know how to do it and don't think it's worthwhile". – Reid Aug 25 '15 at 0:27
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    Fair enough @Reid - I know I take my wheels out easily and quickly without a workstand (I had wheels out on 2 different bikes yesterday between arriving home and making dinner) but I suppose that it might not be the case for all. The "risk" I am talking about is of either contaminating or scratching or bending your rotors, and of scratching or cracking your brake pad. You mentioned higher up: "if you believe the answer is "it can't be done" please write an answer to that effect" - and that is what I have done here - so perhaps the down-vote is a little harsh. – brendan Aug 25 '15 at 23:24
  • @brendan: Thanks for the thoughtful reply and for not adding to the hostility this question is attracting. You may be right. If you don't mind, I'd like to try editing your comment later into the answer in a way I'm comfortable upvoting. – Reid Aug 26 '15 at 16:05
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As I was changing a set of brake pads today, I was reminded of this question and realised that there is a safe way to change the pads and reset the pistons without removing the wheel and that is to unbolt the brake calliper from the fork/frame, reset the pistons and then bolt it back on.

How much hassle this is will depend on the mount required to fit your calliper. If the rotor size/calliper mount is such that the calliper bolts directly to the fork/frame without any adaptor, it should be relatively easy. When remounting the calliper you'll have to re-adjust it to be centred around the disk, but you might have to do this after changing the pads anyway.

However, I do think removing the wheel is the simplest and easiest way to do this.

  • Indeed the tech sheets for some disc brake systems (many of the Avid brakes for example) even recommend removing the caliper from the form/frame as the proper way to change pads. – Glenn Stevens Sep 7 '15 at 7:05
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You don't need to remove the wheel; you just remove the caliper - it's only two bolts.

I do it every two months in order to check the pads and clean the pistons and caliper interior with a small brush. I use it to lube the pistons with a small amount of mineral oil and clean them again with a piece of cotton.

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What do you mean by resetting the pistons? Pushing them all the way back in so that they're flush with the callipers?

It can be done simply by bending the rotor a little and using it to press the callipers back in. It's such a little movement that you won't deform the rotor doing so. Parktools (and I suspect others) make a specific tool. I have also used a wooden door wedge.

However, changing the pads requires removing the wheel. A fail safe feature. DO NOT actuate the brakes with the disk removed. That could pop a piston out requiring specialist tools to replace it. If the piston popped out then there would be contamination but that would be the least of your worries.

You cannot get air in the system changing pads.

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Since you need a support point for any lever you migth be trying to use to push the pistons back, doing it with the wheel on the only support point available is your rotor. That not only leaves you with too small of a gap to effectively apply pressure to the pads, but also puts your rotor at risk, like other have pointed out.

JMAR answer is the only alternative, and that has its risks too, because now you need to re-align the caliper, you need to be careful to avoid damaging the brake hose, and if you are using a thread-locker you need to clean the bolts and apply it again, having to wait 24 hour for it to cure.

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