2

Sorry if this has been asked before (if it has, I haven't found it).

I have Shimano hydraulic disk brakes (type BR-M486).

The front one is beginning to make a slight noise even when I'm not using it:

  • A whispering noise like it's touching lightly
  • Happens even when it's cold i.e. when just starting out
  • Sometimes (but usually not) seems fixable temporarily by applying then releasing the brake

Apparently, when I squeeze the brake lever then the brakes close on the disk, and when I release the brake lever the brakes don't open again far enough to completely clear the disk.

When the noise happens it's slight but more-or-less continuous ... it doesn't sound as if only happens over part of the wheel's revolution.

Shining a torch/flashlight through it from behind, I can see daylight on one side of the disk but not the other ... so apparently, one of the pads doesn't retract off the disk. Looking at the rear brake I can see daylight on both sides.

I tried re-seating the wheel (to try to ensure it's sitting equally on both forks), which made no difference.

Do you (might I) expect a symptom like this if it's been too long since it was last serviced?

I don't understand the internals of hydraulic brakes: I can change a tire but haven't touched brakes. Is it something I could maybe fix, e.g. a screw to turn to adjust it, or should I take it to the bike mechanic?

Maybe this is the service manual for the brakes. Reading it, I see it says (towards the end),

  1. Remove the pad spacers, install the wheel, and then check there is no interference between the rotor and the caliper. If they are touching, adjust while referring to "Installation of the caliper".

Dare I loosen the "caliper fixing bolts", so that the caliper can move sideways? Or (instead of the rotor being too far to one side) is the problem that one of the pads doesn't retract properly, for some reason, so there's simply not enough total space between the pads for the disk?

  • Is the disc true? – Batman Oct 1 '16 at 21:10
  • I'm not sure how to tell that. It makes a slightly louder rubbing/touching noise at one part of the wheel's rotation (which suggests that it's not perfectly true), but I can't see any wobble (though my eyes aren't perfect) when I spin the wheel (which suggests that it's close to true). – ChrisW Oct 1 '16 at 21:21
  • Then I'd start by iterating steps 1 and 2 in the manual (loosen fixing bolts, depress brake lever to clamp rotor, tighten) a few times to see if it fixes things. – Batman Oct 1 '16 at 21:25
4

You should indeed start by just loosening the caliper mounting bolts and trying to do a simple adjustment of the brake. Try to center the gap either by eye or by squeezing the lever while tightening the bolts back down, which sometimes works great and other times not at all. Tighten the bolts to about 8nm, which is firm but not super tight.

If that doesn't work, remove the wheel, remove the brake pads (careful never to touch the pad surface), clean around the pistons with alcohol and a rag, then stick a tire lever in and push the pistons firmly back in place. You've now reset the automatic centering of the brake. Put the wheel back in and re-adjust the brake. You have to work the lever a couple times until it firms up.

If none of that works, you probably have a sticky piston. If it's fixable, it's done by re-lubricating the piston seals with Shimano mineral oil.

  • Thank you. The video "How To Stop Your Disc Brakes Rubbing" agrees with you, saying that re-centering the caliper is the first thing to try (the second, if that fails, is to re-spread the pistons which involves removing the pads). I also found this video which gives me a good look at the internals of a caliper, if it comes to that, – ChrisW Oct 2 '16 at 0:11
  • Two relevant things that would be easy to spot with some more experience are whether one piston is inordinately far extended, and whether the pad gap is narrower than usual. Spreading the pistons is super simple, don't worry about trying it if it seems useful. – Nathan Knutson Oct 2 '16 at 4:15
  • The pads are held in place by a split pin. If I un-bend the split pin to remove it, can I reuse the same pin when reassembling it ... or will I need a spare/new pin (are pins use-once)? – ChrisW Oct 3 '16 at 18:35
  • Those can be straightened and re-bent more or less indefinitely. – Nathan Knutson Oct 3 '16 at 18:54
0

I found a thin (so thin that it's flexible) clean kitchen knife, slid that between the pad and disk, and wiggled it a bit. That apparently stopped the noise for now. Maybe there was some debris or build-up.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.