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  1. #1
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    SLX666 Rear brake doesn't grab.

    I just bought a set of new SLX666s to get rid of my Avids.

    I installed them and immediately did the "moto style" line swap to put the front brake on the right side.

    The front brake feels fanatic - love it. Great modulation and feel; spot on - I didn't even bleed it.

    But the rear brake just isn't strong. What I mean is, I can grab the brake lever and just keep pedaling. All it does is add some resistance, but doesn't not LOCK down onto the rotor. This seems to be more of an issue than a good bleed. At no point, no matter how hard to pull the lever, will it lock the wheel under a load.


    I bled the brake, two different times using the typical gravity method.
    Removed pads, washed them with lacquer thinner to remove any possible oil.
    Sanded the pads
    Washed the rotor with lacquer thinner
    Sanded the rotor


    Nothing. Still the same weak result.

    I'm using the OEM Avid rotors. I figured since they worked with the Avids, they should work fine with the Shimano setup.

    What would cause this?

    I also tried to reverse bleed the brake, but I just couldn't - the syringe wouldn't budge (yes after cracking the bleeder screw). Do the Shimano calipers have a one-way check valve for a bleeder?

  2. #2
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    R&D troubleshooting notes:

    I have a Shimano rotor I'm going to install, and I'm going to install the front pads to the rear and see if those two items do anything.

    Then I have another master cylinder (older BRM-525) I can try too, to see if I can isolate the issue somewhere. (but I assume there has to be someone out there with way more brake-experience than me...?)
    Last edited by KGAmoto; 10-15-2012 at 07:50 PM.

  3. #3
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    Quote Originally Posted by KGAmoto View Post
    R&D troubleshooting notes:

    I have a Shimano rotor I'm going to install, and I'm going to install the front pads to the rear and see if those two items do anything.

    Then I have another master cylinder (older BRM-525) I can try too, to see if I can isolate the issue somewhere. (but I assume there has to be someone out there with way more brake-experience than me...?)
    Go to Shimano's tech website. They have very good tech docs that will guide you through the proper way to set up and to bleed your new brakes.

  4. #4
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    I'll post what I figured out in case others have the same issue down the road:

    I swapped out the OEM Avid rotor with a Shimano that I had (cleaned and sanded both). Still the same result. I even put a different master cylinder on just before that - same thing.

    I took the front pads and just did a swap - front to back and vice versa.

    It was the pads.

    Totally don't get it - only sign is that they seemed to form a "glaze" whereas the fronts don't. The are the SLX Shinano "finned" pads and they look fine, although the edges are rounded as if they were sanded down a bit (I bought this SLX brake set as a new "take off" on eBay, so maybe there was a bit more use on them that told).

    Either way, I'm going to go buy a new set of pads today, put everything back the way it was and hope that does the deal.

  5. #5
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    Quote Originally Posted by KGAmoto View Post
    I'll post what I figured out in case others have the same issue down the road:

    I swapped out the OEM Avid rotor with a Shimano that I had (cleaned and sanded both). Still the same result. I even put a different master cylinder on just before that - same thing.

    I took the front pads and just did a swap - front to back and vice versa.

    It was the pads.

    Totally don't get it - only sign is that they seemed to form a "glaze" whereas the fronts don't. The are the SLX Shinano "finned" pads and they look fine, although the edges are rounded as if they were sanded down a bit (I bought this SLX brake set as a new "take off" on eBay, so maybe there was a bit more use on them that told).

    Either way, I'm going to go buy a new set of pads today, put everything back the way it was and hope that does the deal.
    Sounds like previous owner may have contaminated the pads somehow. Once oil of any sort soaks into a set of pads its real difficult to get rid of it, and sanding/cleaning the surface isn't usually enough. I think getting a new set of pads is a wise move. Make sure you match the compound of the front ones, as Shimano do finned pads in both resin and sintered for those brakes. Read the code off the back of the pads so you know what to get - F01A are the resin finned ones, and F03C are the sintered finned ones.

    PS: In answer to one of your earlier questions, yes the new SLX can be bled either way, but you do need a bit of force to push the oil from bottom to top, so it helps having the proper Shimano bleed kit, which has a small plastic gizmo which clamps the hose to the bleed nipple on the caliper (at the bottom of the picture below), so it doesn't come off and spray oil everywhere when you're pushing hard on that syringe!


  6. #6
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    Thanks for the tips Perth. I actually did figure out the back-bleeding technique. I've done it before, but for whatever reason with this SLX caliper / bleeder, you have to REALLY open up the bleeder to allow fluid through. I was also surprised how dirty the fluid was - these brakes are visually brand new in every way. Seemed odd.

    (and oh - regarding the Shimano bleed kit - I just ask the pharmacist at Target for a small plastic syringe and used that with some clear tubing. Worked perfect.)

    And yeah after a bit of research I saw the resin and metallic options - I plan to buy new pads for both front and rear to start out as "new" as possible and of course the same material.

  7. #7
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    Another update:

    I installed the new pads (I bought Resin, without the cooling fins). The front brake now needed adjustment to give it more play, but the rear brake is STILL off its game. The on-the-fly adjustment screw has to be ran all the way in, and it just does not feel as solid. A little mushy and not as much strength in braking power (and now it squeals a bit, but that will probably go away, I hope).

    I'm just going to go ahead and ride it this weekend to see - but I think the last thing I need to try is to swap calipers.

    Would the (much) longer line of the rear brake give that "lesser" feel when compared to the front?
    Last edited by KGAmoto; 10-19-2012 at 01:59 PM.

  8. #8
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    Quote Originally Posted by KGAmoto View Post
    Another update:

    I installed the new pads (I bought Resin, without the cooling fins). The front brake now needed adjustment to give it more play, but the rear brake is STILL off its game. The on-the-fly adjustment screw has to be ran all the way in, and it just does not feel as solid. A little mushy and not as much strength in braking power (and now it squeals a bit, but that will probably go away, I hope).

    I'm just going to go ahead and ride it this weekend to see - but I think the last thing I need to try is to swap calipers.

    Would the (much) longer line of the rear brake give that "lesser" feel when compared to the front?
    The rear should be just as firm as the front - longer hose length would make an imperceptible difference. And by the way, the adjuster knob on the lever is only to adjust distance lever is from the bar, it makes no actual difference to bite point of the brake (only XT and above have bite point adjustment).

    However, things starting to add up here - dirty oil, mushy feel, contaminated pads - all would point to there being a leak somewhere in the calliper end of your rear brake. Would suggest checking:-

    1. Where hose goes into banjo fitting - there was a batch of shimano brakes about a year ago with bad crimps, and as yours are second hand it may be one of those.

    2. Dirt in the oil suggests there may be a leaky seal on one of the pistons - previous owner could have damaged a seal pushing piston out too much in a bleed operation gone wrong.

    If you still can't find the problem, and tho' it pains me to say it, as I'm all for home maintenance, it may be time to take it to the LBS for them to sort out.
    Last edited by PerthMTB; 10-19-2012 at 06:29 PM.

  9. #9
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    Quote Originally Posted by PerthMTB View Post
    The rear should be just as firm as the front - longer hose length would make an imperceptible difference. And by the way, the adjuster knob on the lever is only to adjust distance lever is from the bar, it makes no actual difference to bit point of the brake (only XT and above have bite point adjustment).

    However, things starting to add up here - dirty oil, mushy feel, contaminated pads - all would point to there being a leak somewhere in the calliper end of your rear brake. Would suggest checking:-

    1. Where hose goes into banjo fitting - there was a batch of shimano brakes about a year ago with bad crimps, and as yours are second hand it may be one of those.

    2. Dirt in the oil suggests there may be a leaky seal on one of the pistons - previous owner could have damaged a seal pushing piston out too much in a bleed operation gone wrong.

    If you still can't find the problem, and tho' it pains me to say it, as I'm all for home maintenance, it may be time to take it to the LBS for them to sort out.
    Yeah that's what I thought in regards to the feel of the back brake in relation to the front (I've had the low-level Deore BR-M525s before that felt even better than these). And like you suggest, I'm starting to culprit the caliper itself and / or the banjo fitting. I did check the tension of the banjo bolt, but the crimp may be the issue as you're eluding to. I'll dig into that. I do plan to either swap the calipers, or install that M525 caliper if I can (but does not have the banjo fitting style, but I can swap the line and caliper all in one - thats a good idea actually. If that old M525 caliper feels better / correct, then that is my ... well almost my answer. Still have to figure out if its the line or caliper).

    Thanks for reading into my posts in-detail.

    I'll figure it out; I'm really mechanically savvy and should be able to get it. Just don't have much experience with troubleshooting brakes; the Shimanos I've had in the past were perfect. Thinking I should have sprung for a new XT set instead....

  10. #10
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    Good luck! Its a pain when you can't pin down what the problem is, but I've got both SLX and XT and when they're working right they are great brakes!

    PS: Here's alink to the thread about the banjo crimp problem XT-XTR BR-M785 Warranty Brake Leaking Issue

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    Hello, I am new and I will use this post to post question if not a problem, 'cause I still can't open new thread...

    So, I also bought new SLX666 and feels great, my both brakes are good and strong, but what I noticed with my rear one is that I can pull the lever all the way to the grip if I really press it strong (has less resistance than my front brake). I feel like it might be insufficient mineral oil problem? I am hydraulics newbie atm, so if that is the problem, can I pour more oil in it or does it need complete rehaul (I dunno what's ENG term for removing air and refilling oil process ) with oil?

    Thx!

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    Hi Raxik,

    Not a problem. Your lever shouldn't go to the bar no matter what, unless you have the adjuster screw all the way out but even then it should not.

    I would just bleed the brake first. Go find a small plastic syringe that you should be able to find at a pharmacy and some clear rubber tubing (you can get that at a pet store - I used stuff that it meant to be used as air line for fish tanks). Or, you can buy the overpriced Shimano 50ml of Mineral Oil that also comes with a piece of clear tubing.

    Remove the cap from your master cylinder at the bar - fill your syringe up with mineral oil (attach the clear tubing to the syringe first). Remove the rubber cap that is over the caliper bleeder screw. Connect the syringe / rubber tubing. Open the bleeder (you'll have to open it quite a bit). Push the syringe until you begin to see fluid coming into (and probably overflowing) the master cylinder at your bars.

    That's my favorite way to bleed. Try that first.

  13. #13
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    Quote Originally Posted by PerthMTB View Post
    Good luck! Its a pain when you can't pin down what the problem is, but I've got both SLX and XT and when they're working right they are great brakes!

    PS: Here's alink to the thread about the banjo crimp problem XT-XTR BR-M785 Warranty Brake Leaking Issue
    Hey Perth - I went out today and put in a good 25 miles and after a ton of downhill, the rear brake feels really good. The tell-tale issue was, I could be cruising around in my backyard, grab the rear brake and there was no way it was going to lock up. I could just keep pedaling (like I said before).

    Now today after hammering for a long ride, they're both pretty good. The front still activates sooner, but I can obviously just even that out with the adjusters and its decent. The back wheel definitely locks up and firmly grabs as it should.

    But regarding that thread you posted earlier, this DID happen - the lines were leaking but after long descents the oil burnt off basically. We'll see - but I just can't see any signs of a leaking line anywhere.

  14. #14
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    It's the work of the devil. Sorry I couln't resist.

  15. #15
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    Quote Originally Posted by KGAmoto View Post
    Hi Raxik,

    Not a problem. Your lever shouldn't go to the bar no matter what, unless you have the adjuster screw all the way out but even then it should not.

    I would just bleed the brake first. Go find a small plastic syringe that you should be able to find at a pharmacy and some clear rubber tubing (you can get that at a pet store - I used stuff that it meant to be used as air line for fish tanks). Or, you can buy the overpriced Shimano 50ml of Mineral Oil that also comes with a piece of clear tubing.

    Remove the cap from your master cylinder at the bar - fill your syringe up with mineral oil (attach the clear tubing to the syringe first). Remove the rubber cap that is over the caliper bleeder screw. Connect the syringe / rubber tubing. Open the bleeder (you'll have to open it quite a bit). Push the syringe until you begin to see fluid coming into (and probably overflowing) the master cylinder at your bars.

    That's my favorite way to bleed. Try that first.
    Thx alot m8, will try that, let me just ask, if I fill it completly with oil till it bleeds it out, will the break go too stiff?

  16. #16
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    Quote Originally Posted by Raxik View Post
    Thx alot m8, will try that, let me just ask, if I fill it completly with oil till it bleeds it out, will the break go too stiff?
    No. The amount of oil in the master cylinder will never affect the feel (unless its empty).

  17. #17
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    My rear brake is back to it's poor-working state. Again. I pulled the pads out tonight - there is traces of oil around the edges of the pads and the pistons in the caliper.

    Something is leaking.

    I read through the post given above about the batch of fittings that weren't crimped properly on some lines, but I can't see ANY evidence of fluid leaking from the fitting.

    Was there a batch of bad calipers too?

  18. #18
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    Check the piston seals...

  19. #19
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    New brake pads always need 'bedding-in'. You pedal up to speed and then brake hard until you have almost stopped completely. Do not lock the brakes. Do that 20 times then repeat for the other brake.

    Leaks from piston seals can cause confirmation of the pads which will make the pads squel and I would think will make them less effective at stopping too. Remove calliper from the mounting bolts, remove the pads and using a clean rag wipe around the seals.

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    Quote Originally Posted by chinaman View Post
    New brake pads always need 'bedding-in'. You pedal up to speed and then brake hard until you have almost stopped completely. Do not lock the brakes. Do that 20 times then repeat for the other brake.

    Leaks from piston seals can cause confirmation of the pads which will make the pads squel and I would think will make them less effective at stopping too. Remove calliper from the mounting bolts, remove the pads and using a clean rag wipe around the seals.
    Bedding in the pads wasn't necessary; I know what you're saying, but for a bicycle, it doesn't do anything. The front brake has always been spot-on.

    It must be a piston seal leak for the back brake. I have to look at see if the seals are replaceable, which I assume. That has to be it; yesterday toward the end of my ride, my brakes were excellent - must have burnt the oil off. Once it sits for a few days, they're back to the point of not working again (again, like I've said, you can literally grab all the brake you possibly can, and keep pedaling).

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