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m785 xt brake issue/ advice sort

swod1swod1 Posts: 1,639
edited May 2019 in MTB workshop & tech
Been using m785 xt disc brakes for the last couple of years, never had a problem but recently I cant seem to get rear pads to bed in properly and so got the dreaded squeal.

Front is absolutely spot on.

I've noticed the rear when you press the lever there is like a clicking noise coming from it and the pistons don't retract as quick as I thought they would.

I've done the usual retract pistons out a little and clean etc. disc isn't warped and I've got the caliper aligned centrally.

Its not a spongy lever feel so what I'm thinking is fit the bleed block and pass through some new fresh oil, I've just ordered the brake bleed funnel and already got a bleed kit at home I bought but never needed to use until now.

Is this worth a shot before buying a new brake/caliper?

Anything else to try?
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  • JBAJBA Posts: 2,852
    A bleed can't make it any worse, so go for it.
    Another thing to check is that the metal spreader between the pads isn't bent and catching the rotor.
    “Life has been unfaithful
    And it all promised so so much”

    Giant Trance 2 27.5 2016 ¦ Sonder Broken Road 2021¦ Giant Revolt Advanced 2 2019 ¦ Giant Toughtroad SLR 1 2019 ¦ Giant Anthem 3 2015 ¦ Specialized Myka Comp FSR 2009
  • billycoolbillycool Posts: 833
    Well worth chucking some new oil through to see if that helps.

    I had the same brakes before and found them very good.

    Are you saying that the brakes seem to still work okay but the pads just aren't bedding in? Are do you think you're losing some braking power? Is the clicking coming from the lever?

    I think you might need to go through a process of elimination first and it really depends what you think the underlying issue might be. I'm not sure if a faulty lever/caliper should cause the pads to not bed it (unelss you aren't getting the braking power and they are effectively just dragging/glazing). Have you changed pads?

    Based on my own personal brake experiences, these things fixed various issue over time:

    - bleed and flush through new oil
    - new pads
    - new rotors (only on close inspection did I realise how worn mine were)
    - check all nuts/bolts are tight
    - check the spring in between the pads
    - buy new brakes :D

    IF the lever has an issue with the internals, it might need some TLC or replacement.
    "Ride, crash, replace"
  • swod1swod1 Posts: 1,639
    yeah it is weird as the front is fine no issue but rear is awful I think it needs a bleed or something.

    Can mineral oil go off after a while of use in the hose?

    Never had an issue bedding in pads before I'm using semi metallic uberbike pads in rear what I had available at the time and race matrix in front.

    definitely clicking come from the brake lever I think but will narrow it down. I think the pads are glazing on disc as the pistons not retracting quick enough.

    I'm using cheap steel discs off ebay rear is 160m and front 180m, same disc on front but just larger size, they aren't worn either.

    I was hoping they would last a while longer as I am fancying some new hope tech 3s but didn't want to buy them yet.

    hopefully new bleed funnel arrives at weekend so I can spend some time and sort the problem out with having Monday off work for a day to fiddle about with it.
  • The RookieThe Rookie Posts: 27,809
    No mineral oil can’t go off.
    Current steed - Whyte T129, 2013 frame, mongrel Revelations, Giant dropper, Stans S1 wheelset. 12, Magura Trail Sport brakes, 1x11. 12.8Kg
  • swod1swod1 Posts: 1,639
    The Rookie wrote:
    No mineral oil can’t go off.

    I thought so, I'll give it a bleed over the weekend and see what happens and report back.
  • billycoolbillycool Posts: 833
    I bled my SLX recently and was surprised by how much black crud came out. I think it was the result of some of the seals failing as my lever keeps going soft. I've got one more thing to try and then it's getting replaced.

    Let us know how you get on.
    "Ride, crash, replace"
  • swod1swod1 Posts: 1,639
    well I passed some mineral oil through and fluid was still pink which was a good sign.

    Looks to be more down at calliper end it’s working a little better.

    Didn’t see any air in calliper and was definitely none in the lever end.

    As you look towards the bike from the back with the wheel off the right hand piston in calliper doesn’t move as fast as the left.
  • The RookieThe Rookie Posts: 27,809
    That’s perfectly normal.
    Current steed - Whyte T129, 2013 frame, mongrel Revelations, Giant dropper, Stans S1 wheelset. 12, Magura Trail Sport brakes, 1x11. 12.8Kg
  • swod1swod1 Posts: 1,639
    The Rookie wrote:
    That’s perfectly normal.

    Ah great, that’s good to hear.

    Will keep an eye on it, the front has never been an issue.

    Need a better syringe one in bleed kit I have is awful so stiff to push the fluid through.
  • billycoolbillycool Posts: 833
    The fluid never pushes through that quickly anyway. Also make sure the bleed nipple is open enough.

    To test your syringe - just fill it with some oil and squirt it back into the bottle. If it is super slow or too firm then you might need a new one. If it works well then it's ok.
    "Ride, crash, replace"
  • AlejandrosdogAlejandrosdog Posts: 1,975
    Is it possible to rebuild these if the seals perish?
  • mark~pmark~p Posts: 55
    I would not have thought bleeding the brakes would make any difference as the squeal is the pads/rotor vibrating.
    Take the pads out and rub them on some flat sandpaper to remove any surface film that may have built up.
    You can also spin the wheel with some fine emery on the rotor.

    Have you inadvertently got the rotor/pads contaminated with oil, polish or the gloop that is sprayed on suspension. That final one is a killer as it usually has some form of silicon in.
  • billycoolbillycool Posts: 833
    Is it possible to rebuild these if the seals perish?

    Not really as Shimano do not do spares.
    "Ride, crash, replace"
  • AlejandrosdogAlejandrosdog Posts: 1,975
    BillyCool wrote:
    Is it possible to rebuild these if the seals perish?

    Not really as Shimano do not do spares.

    Painful.
  • billycoolbillycool Posts: 833
    BillyCool wrote:
    Is it possible to rebuild these if the seals perish?

    Not really as Shimano do not do spares.

    Painful.

    Yup - I knew the calipers were fairly disposable as I broke a piston (me being heavy handed) and I had to buy an ali one from China (still not tried fitting it - bought a new caliper in the end). I only found out recently when one of my SLX levers was losing pressure that I can't get new internals and it's FUBAR'd. Some on-line searching confirmed the lack of spares.

    I've never had issues with Shimano before as they work well, easy to bleed/change pads etc. Just a real shame that if the internals fail, you've got a decent lever with knackered internals. I'll stick with them and accept that every so often I'll need to buy some new bits.
    "Ride, crash, replace"
  • swod1swod1 Posts: 1,639
    mark~p wrote:
    I would not have thought bleeding the brakes would make any difference as the squeal is the pads/rotor vibrating.
    Take the pads out and rub them on some flat sandpaper to remove any surface film that may have built up.
    You can also spin the wheel with some fine emery on the rotor.

    Have you inadvertently got the rotor/pads contaminated with oil, polish or the gloop that is sprayed on suspension. That final one is a killer as it usually has some form of silicon in.

    I checked all that and even undoing the rotor bolts and re tightened them in the star pattern again to make sure the disc is on the hub correctly.

    On the caliper end one of the pistons was coming out too much compared with the other seems to be ok now that I did a full bleed through as normally you can get a way with just doing the lever end with the bleed funnel.

    Are the latest xt m8000 brakes an improvement on the m785 xt brakes? I've seen the set for 122 pound online.
  • billycoolbillycool Posts: 833
    swod1 wrote:
    mark~p wrote:
    I would not have thought bleeding the brakes would make any difference as the squeal is the pads/rotor vibrating.
    Take the pads out and rub them on some flat sandpaper to remove any surface film that may have built up.
    You can also spin the wheel with some fine emery on the rotor.

    Have you inadvertently got the rotor/pads contaminated with oil, polish or the gloop that is sprayed on suspension. That final one is a killer as it usually has some form of silicon in.

    I checked all that and even undoing the rotor bolts and re tightened them in the star pattern again to make sure the disc is on the hub correctly.

    On the caliper end one of the pistons was coming out too much compared with the other seems to be ok now that I did a full bleed through as normally you can get a way with just doing the lever end with the bleed funnel.

    Are the latest xt m8000 brakes an improvement on the m785 xt brakes? I've seen the set for 122 pound online.

    Hard to say - I had the M785's and swapped them out for M8000's, purely because I had the new ones as part of a 2x10 kit I bought and subsequently sold most of it to someone else. Both brakes do what they are supposed to do and can't really tell much difference to be fair. My friend who bought the M785's loves them and says he's not had to touch them in the 9 months he's had them.

    The M8000 are all black and more `streamlined` and therefore look quite nice!

    Other than that - as an average XC rider who does Wales and the Peaks once a year, I can't tell much difference.
    "Ride, crash, replace"
  • JBAJBA Posts: 2,852
    I have M785 on one bike and M8000 on another and can’t tell any difference in terms of performance.
    “Life has been unfaithful
    And it all promised so so much”

    Giant Trance 2 27.5 2016 ¦ Sonder Broken Road 2021¦ Giant Revolt Advanced 2 2019 ¦ Giant Toughtroad SLR 1 2019 ¦ Giant Anthem 3 2015 ¦ Specialized Myka Comp FSR 2009
  • swod1swod1 Posts: 1,639
    Interesting I've never had to service my brakes either until recently when I've been having the rear brake issue. Been solid for last couple of years I've owned them think its actually longer than that.

    Going off topic, billycool - have you tried sjs cycles for spare parts?
  • billycoolbillycool Posts: 833
    swod1 wrote:
    Interesting I've never had to service my brakes either until recently when I've been having the rear brake issue. Been solid for last couple of years I've owned them think its actually longer than that.

    Going off topic, billycool - have you tried sjs cycles for spare parts?

    When you say spare parts - you mean as in bits for brakes (for myself) or buying stuff from them in general?
    "Ride, crash, replace"
  • swod1swod1 Posts: 1,639
    BillyCool wrote:
    swod1 wrote:
    Interesting I've never had to service my brakes either until recently when I've been having the rear brake issue. Been solid for last couple of years I've owned them think its actually longer than that.

    Going off topic, billycool - have you tried sjs cycles for spare parts?

    When you say spare parts - you mean as in bits for brakes (for myself) or buying stuff from them in general?

    I mean for replacement parts for the brakes, used them a few times for spare parts. They stock a very large range of spare parts for shimano, if you can find a shimano part number for the bit you need then you can probably by it from them or someone else.
  • billycoolbillycool Posts: 833
    swod1 wrote:
    BillyCool wrote:
    swod1 wrote:
    Interesting I've never had to service my brakes either until recently when I've been having the rear brake issue. Been solid for last couple of years I've owned them think its actually longer than that.

    Going off topic, billycool - have you tried sjs cycles for spare parts?

    When you say spare parts - you mean as in bits for brakes (for myself) or buying stuff from them in general?

    I mean for replacement parts for the brakes, used them a few times for spare parts. They stock a very large range of spare parts for shimano, if you can find a shimano part number for the bit you need then you can probably by it from them or someone else.

    I've had a look and they do bolts, fixings, bleed nipples etc but none of the internals. AFAIK, Shimano don't do service kits for the internals so you won't find them anywhere.

    SJS is a good resource for other bits though.
    "Ride, crash, replace"
  • swod1swod1 Posts: 1,639
    BillyCool wrote:
    I've had a look and they do bolts, fixings, bleed nipples etc but none of the internals. AFAIK, Shimano don't do service kits for the internals so you won't find them anywhere.

    SJS is a good resource for other bits though.

    Sounds like you'll need to replace the lever then but sometimes worth just buying the whole brake with Shimano as aren't the levers for SLX like 35 pound?
  • billycoolbillycool Posts: 833
    swod1 wrote:
    BillyCool wrote:
    I've had a look and they do bolts, fixings, bleed nipples etc but none of the internals. AFAIK, Shimano don't do service kits for the internals so you won't find them anywhere.

    SJS is a good resource for other bits though.

    Sounds like you'll need to replace the lever then but sometimes worth just buying the whole brake with Shimano as aren't the levers for SLX like 35 pound?

    Yup - I did the same in reverse when I had issues with my caliper. I bought the whole SLX brake new on eBay for £35 and took the caliper off. I now have a brand new lever, but it's the wrong side. And no, I'm not stripping the internals out. It will be kept as a spare or sold to fund new toys (or a lever for the correct side).

    There also seem to be some sensible deals on-line as well.
    "Ride, crash, replace"
  • swod1swod1 Posts: 1,639
    BillyCool wrote:
    Yup - I did the same in reverse when I had issues with my caliper. I bought the whole SLX brake new on eBay for £35 and took the caliper off. I now have a brand new lever, but it's the wrong side. And no, I'm not stripping the internals out. It will be kept as a spare or sold to fund new toys (or a lever for the correct side).

    There also seem to be some sensible deals on-line as well.

    Yeah if they were like hope or another brand you could rebuild them but at the price point its not worth it depending on what's failed.

    touch wood so far so good but looking at replacement caliper for rear I've been tempted by a whole new brake set due to the offers about on them.
  • billycoolbillycool Posts: 833
    swod1 wrote:
    BillyCool wrote:
    Yup - I did the same in reverse when I had issues with my caliper. I bought the whole SLX brake new on eBay for £35 and took the caliper off. I now have a brand new lever, but it's the wrong side. And no, I'm not stripping the internals out. It will be kept as a spare or sold to fund new toys (or a lever for the correct side).

    There also seem to be some sensible deals on-line as well.

    Yeah if they were like hope or another brand you could rebuild them but at the price point its not worth it depending on what's failed.

    touch wood so far so good but looking at replacement caliper for rear I've been tempted by a whole new brake set due to the offers about on them.

    I'm the same. Almost worth buying a complete new set and either selling my old brakes or steal the bits I need and sell the rest.

    I only really need a new lever, but needing and wanting are 2 different things....
    "Ride, crash, replace"
  • The RookieThe Rookie Posts: 27,809
    I've never found a lack of spares an issue, my 2005 M765's are still working as good as they ever did.

    I have Formula's on my main bike, full suite of spares available which is just as well as they are less reliable than the Shimano!
    Current steed - Whyte T129, 2013 frame, mongrel Revelations, Giant dropper, Stans S1 wheelset. 12, Magura Trail Sport brakes, 1x11. 12.8Kg
  • swod1swod1 Posts: 1,639
    Any ideas with this?

    The squeak is back but its strange I put the brake on lightly and its fine can hear the pads abrasion on the disc (semi metallic) but putting the brake on fully but not locking the wheel up there's a like squeak noise.

    One thing I didn't do before fitting the new pads was take some wet n dry emery paper over the disc, I just cleaned it with isopropyl alcohol.

    There is a shine on the disc but I've got similar on the front and that brake has never been any problem.
  • billycoolbillycool Posts: 833
    Disc rotors should just need cleaning and nothing more.

    Pads can do with cleaning and emery paper. I did mine last night as part of my ongoing brake issues.

    I've swapped out my left/rear SLX lever for a new XT M8000 and my right/front will get the same treatment tonight. If that doesn't work, I'll look at the calipers.

    Brakes are great when they work but a pain when you are chasing squeaks and squeals that shouldn't be there.
    "Ride, crash, replace"
  • swod1swod1 Posts: 1,639
    BillyCool wrote:
    Disc rotors should just need cleaning and nothing more.

    Pads can do with cleaning and emery paper. I did mine last night as part of my ongoing brake issues.

    I've swapped out my left/rear SLX lever for a new XT M8000 and my right/front will get the same treatment tonight. If that doesn't work, I'll look at the calipers.

    Brakes are great when they work but a pain when you are chasing squeaks and squeals that shouldn't be there.

    That's what I thought, I'd put in a brand new pair of pads as I buy them in packs of 4 and ridden my bike to work all last week to bed the pads in etc.

    It's weird as its been fine but then back again. Been pricing up and slx caliper is only 30 quid so not too bad to replace if I need to.

    wondered if to go back to sintered pads again, never any issue when first fitted the brakes as metal pads came in them from crc but make too much noise when wet.
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