Hi there, I was wondering if anyone would be able to suggest anything here. I am a bit of a dunce with brakes and I am struggling to google. Furthermore I'd like to treat taking it to the shop as a last resort!
I have Shimano m7120 SLX 4 pots on my enduro bike, new from late 2023. I cannot for love nor money stop the front from rubbing, which started a month or two ago and seems to be getting worse.
I have bled it thoroughly, made sure the rotor is dead flat (rotor is from early 2024), but I still get a constant brake rub. It got so bad on a night ride last week that there was a constant high pitched whine, and it ruined the ride for me. Weirdly, it goes away if I hold the lever and pull it ever so gently, but its like 1 degree of lever movement where this works. Or if I pump the brake it goes away for a second and then slowly starts to build up again. But that's not really a sustainable way to ride.
Pads in decent condition with plenty life left, as far as I can tell there's no contamination. I think a piston is pushing back in against the rotor more than it should as aligning it doesn't seem to work. I've had the top bleed port open and pushed the pistons back to remove any excess fluid and it still seems to happen. The pistons are no harder to push back in than I have experienced with any other functional brakes.
Is this a bike shop job? feel like I have exhausted my meagre knowledge but I would really welcome any ideas. I've heard about the thing where you oil up the piston a bit and work it through it's motion a few times?
Take the pads out, wheel off, and you'll be able to see if a piston is sticking when you press the lever.
You an then extend it and lubricate the sides with a dab of brake fluid. If it's really sticking you need to hold the other ones down and press lever to get it moving.
If all looks ok then it's likely just alignment. You need the caliper centred over the rotor, which sounds obvious. But with wonky pistons people often centre between the pads, which is not the same thing and exacerbates alignment problems.
@HoratioHufnagel - haha, I think that would be a fast track to me giving up on mountain biking altogether. I cannot stress how annoying that sound is. Sounds like tinnitus.
@Garry_Lager - thanks for that. I think last time I noticed that all of the pistons come out, but one (I think the rear outboard one) doesn't retract as well. I might drain the entire system and do all of those steps incl. dabbing some mineral oil on the outside of the piston (s). It's possible I am just terrible at bleeding and haven't freed up an air bubble somehow. If that doesn't clear it then it's off to the bike shop I suppose. Annoying because it's only 1.5 years old and that's the bike I use least for the odd weekend or evening jolly, not like it has thousands of miles on it it.
Any flex or movement in axle or bearings in hub?
but one (I think the rear outboard one) doesn’t retract as well
Brake pistons don't retract like that. When you apply the brakes, the seals flex as the piston moves out. When you release them, the seals return to their normal shape and that's what retracts the piston. When the pads wear a bit, the flex becomes too much and the piston slips though the seal a tiny bit, that's why hydro brakes are self-adjusting.
IME, the best way to stop a brake from rubbing is to find a big hill and drag the brake all the way down. That's assuming that the rotor isn't warped. If the rotor's warped, you'll never stop it rubbing, you need a new rotor.
Annoying because it’s only 1.5 years old and that’s the bike I use least for the odd weekend or evening jolly, not like it has thousands of miles on it it.
This is more likely to create issues with brakes, ime. It's the ones you ride the balloches off that work flawlessly, but the bike that gets ridden occasionally will be prone to disk brake issues.
Like what thols2 is saying a hard ride with plenty of heavy braking can settle a brake down nice - if you have it set up ok then smash it down some hills.
@thols2 - shows how much I know. I always just assumed that the pistons retract from the "vaccum" created from releasing the level.
It's weird that it goes away after braking and then comes back gradually over 5 seconds or so. I suppose the rotor could be warped beyond my perception but it looks dead straight to me and sits flat on a level surface. Its not a "clang, clang" type noise that I've had before, its a constant whine.
Been mountain biking for years...you should see me try and bleed SRAM brakes for some comedy. Just can't get my head around stoppers for some reason.
Could be conical washers. Replace with flat washers and you remove one of the plains of adjustment. Could also give rotor shims a go to help dial in pad alignment
Suspect it goes away after braking as the pads are still contacting the rotor hard enough to not squeal and as they retract the squeal starts when one ends up just lightly rubbing the rotor. Lightly pulling the leaver probably engages the other one enough to make that start squealing.
As above, pads out and get a piston tool from r3pro so you can extend them one at a time to exercise them without popping them out. Also clean with IPA and make sure they are all pushed fully home before centering calliper over the rotor. As above, lining up using pads creates problems. Plenty of people lube the pistons but certainly SRAM advise against this. As above again, it’s the seals moving that retracts the pistons not the pressure moving the pistons past the seal so a lubricant on the pistons makes them slip past the seal.
Eventually decided to buy a new rotor after all the above didn't work and lo and behold the problem is solved. Must have been warped in a way that is totally undetectable or visible.
Yes, they only have to be warped by a fraction of a millimeter to rub slightly.