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Shimano SLX M655 Caplier issue

So, I bled my rear brake last night but the lever movement didn't improve. It's perfectly usable, but annoying. I can, with full strength get it all the way back to the handlebar (just).

Anyway, I was playing around with the caliper this morning. Adding a little mineral oil around the pistons just to lube them a little in case they were sticking at all, and one of them popped out (when pulling the lever). Mineral oil proceeded to drip onto the floor. I'm freaking out at this point (i'm off to Snowdonia tomorrow). So, I get it back in, and then there is no resistance on the lever at all. I re-bleed and follow all the usual stuff, bubbles coming out of the reservoir etc. In fact I got the lever feeling really super responsive and firm after that. I finished up, got the pads in, wheel on and took it down the road, but now the lever feels pretty much exactly like it did before. Where's there firmness and responsiveness gone from when I had the yellow block in the caliper?

Also, have I broken the caliper by being an idiot and letting the piston come out, or will it be ok? It didn't hit the floor or anything fortunately.

2007 Merlin Malt 4


  • reaperactualreaperactual Posts: 1,143
    edited September 2021
    There's no reason to assume your caliper won't work as it did because you popped a piston. Is the piston in question moving okay now, as in you pushed it in evenly/straight and haven't got it stuck in the bore?

    Are you talking about a full caliper up bleed after the piston went back in? Could need another go as maybe there's some air still trapped in the caliper itself?

    Have you checked lever, piston seals and hose connections for any sign of leaks?

    Before anything else I'd do another thourough bleed using all the tricks to ensure you get as much air/bubbles possible out and perhaps even a long or overnight purge to finish off.
  • Yes, the piston appears to be moving in and out okay.

    Re the bleed I did, I syringed mineral oil in through the caliper nipple and syringed it out of the lever reservoir. Apart from the normal pin censored sized bubbles you get when bleeding, there were a few proper big bubbles coming out, which I guess was to be expected having popped and piston.

    No signs of any leaks from the lever or hoses, and 'I think' the piston seals are okay.

    I think it's definitely worth doing another full bleed. I'll angle the bike on the stand so the hose is heading as vertical as possible to help with any trapped bubbles.

    Excuse my ignorance, but what is a long/overnight purge?
    2007 Merlin Malt 4
  • reaperactualreaperactual Posts: 1,143
    edited September 2021
    If I'm trying to do the best bleed possible I'd normally totally remove the caliper and hoses from the frame to get properly vertical with minimum bends. This lets you move the caliper in lots of angles while pushing fluid through.

    I use hair clippers touching the hose to vibrate and encourage air bubbles out which all seem to help the process.

    A purge is basically leaving the funnel with fluid in the lever bleed port while using an elastic band/strap to pull the lever as you would for braking and leave for a good length of time or even overnight if possible.
  • I'll do that that then. Will the caliper bolts need torqueing when I put it back on the frame?

    Hair clippers are a VERY good idea. I've just been tapping it with screwdriver handle.

    Ah, not sure that method would work on mine. They're the old skool brakes from about 2008/09 where the whole reservoir cap comes off with 2 screws.
    2007 Merlin Malt 4
  • reaperactualreaperactual Posts: 1,143
    edited September 2021
    Ideally you'd torque the bolts back up if you have a torque wrench to around 8Nm. I'd say the force of opening a stiff door handle as a rough comparison.

    Hair clippers were there at the time but anything that vibrates will do.

    My bad with your lever type but I guess you could still do a purge of sorts except make sure to check and top up fluid that may replace any tiny bubbles you get out.

    Then again I reckon
    you could leave the purge seeing as you are riding tomorrow and see what results you get with the other methods Pal.
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