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I have a pair of second-hand Shimano ST-RS685 hydraulic brake levers / shifters. Unbeknownst to me they hadn't been used for a year and had quite a bit of dirt / grit inside, which I've cleaned to a degree (as much as I can without taking them apart).

When pulling the brake lever all the way back to the bar, it often does not return back and stays in that position. There is a cylinder thing that is pushed by the lever when you press it which moves along a channel with a slight kink in it (which ultimately pushes the fluid out of the reservoir to the pads).

When I release the lever, depending how far I've pushed it, this cylinder will either get stuck at the far end or move back very slowly and not all the way as it should. It often gets stuck at the kink. The lever also gets stuck as I think it's the cylinder thing going back that pushes the lever back. If I've not pulled it all the way, then often moving the lever back manually will allow the cylinder thing to go back to its resting position. However, if it's at the far end then moving the lever back often has no effect.

The first image shows the lever pulled back all the way with the cylinder thing at the far end of the channel (circled in red). I've also added a yellow arrow pointing to the bit that comes from the lever that pushes the cylinder thing.

Lever pulled to the bar

The second image shows how it should normally look when in a resting position.

Lever resting position

I haven't bled that brake yet, but I have on the other brake and that one does it as well, but nowhere nearly as bad, although it does mean that sometimes the pads slightly bind. Bleeding didn't seem to have any effect on it.

I've tried cleaning the levers as they did seem to have some grit / dirt in them and also followed this guide:

(How To Service Your Shifters - GCN) which works for about a day or two. Then the levers seem to seize up again.

Edit: I'm still having problems. Not as serious as before since I've cleaned up the levers as much as I can, but the cylinder thing in both levers will not return fully (leaving a gap of about 15%-30% between the resting position in the channel and the cylinder), dependent on weather (I think).

Yesterday, it was about 25C here and I'd left the front of the bike towards the front of my garage where it gets quite warm. Both levers were stuck at with about a 30% gap, as described above, and while I could manually push the cylinder back, this didn't stop it getting stuck again the next time I pulled it. The bike was unridable as both wheels' brakes were being applied a fair amount. Later on in the evening, when it had cooled down considerably, they were working fine, leaving virtually no gap on releasing the levers.

Today, it was hotter, but I'd put my bike in a cooler spot. The result was that they both were getting stuck with about a 15% gap. Bike would have just been ridable but with a lot more resistance than normal. Later on when it cooled down, again this problem had largely resolved itself.

Has anyone encountered this?

Further edit

I cleaned them again with compressed air and put PTFE lube in the levers (Triflow) and then greased the moving parts. Thought they were behaving better, but they're doing it again today (24C and sunny). Again getting stuck at about 30% in the channel.

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    The problem may be inside the hydraulic cylinder rather than the external lever mechanism. Possibly you will need to detach the hose, take the reservoir cover off and try to flush any contamination out of the cylinder. – Argenti Apparatus Jul 17 at 13:28
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    @ArgentiApparatus that doesn't explain why doing the method from the video works for a bit. Even when it's seized up again it's not as bad as it was when I got them. Also since it's affecting both levers (albeit not as bad on the other one), it seems very unlikely that both hydraulic systems would be contaminated. Although I have yet to bleed the worse one so maybe bleeding would help. – CuthbertAnnihilator Jul 17 at 13:42
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    Weird problem. I'm curious what happens when you actuate the brake lever without any fluid in there at all and with the hose disconnected. Seeing if it moves freely then would give you some clues. I'd be worried something happening to them like DOT fluid exposure. Also check for piston damage, always a good thing to do when Shimano road hydros act up since it's common. You're looking for crunchy sensations when you push on the pads/pistons. Blowing out the pistons to get a visual check while also cleaning inside the calipers could also be prudent. – Nathan Knutson Jul 18 at 7:37
  • Having realised I can access the back of the piston by pulling the lever, I've cleaned it up a bit in that location and also in addition to the tips from the GCN video, used some proper grease on any moving parts I can access with a long thin screwdriver and not just the spray lubricant. A few hours after doing that it seemed to be a lot smoother and still seems to be today, so hopefully that's done it. @NathanKnutson I don't think it's piston damage since it was doing it with no fluid in the system at all. – CuthbertAnnihilator Jul 18 at 22:46
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    I believe that funny shaped slot is the “servo-wave” mechanism. It changes the lever ratio as you go along to give a shorter free stroke at the beginning, but then precise and modulate-able braking near the end of its stroke. – MaplePanda Jul 31 at 22:34

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