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I have a Trek 3900 mountain bike with disc brakes, and I just took it out of winter storage to start riding it for the season.
Everything works well, except for one very annoying problem: when I stop the disc brakes make an extremely loud obnoxious squealing sound. Any ideas of what is causing this?
Thanks

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Check your fork for play in the headset. Had that influencing the brake once. –  posipiet Apr 10 '11 at 17:46
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4 Answers

up vote 5 down vote accepted

The easy thing to check is whether the pads have got a little damp while you weren't riding it. The solution there is to ride it a bit. If the noise bothers you try riding with the brake slightly on to heat it up.

Unfortunately that conflicts with the other problem: if there's oil on the rotor you should clean it immediately and avoid using the brake until you can clean it (to avoid getting oil in the pads because they're hard to clean). Wipe the rotor with either disk cleaner or rubbing alcohol and a clean cloth. Keep doing that with new solvent and new cloth until no further dirt comes off the rotor. Then take a quick test ride, using the brake. Ideally pedal hard with the brake on, so you heat the pads and rotor (and also get an idea of whether the brake works). Remember that riding with the brakes on heats up the rotors - be careful touching them immediately afterwards. Wipe the rotor with a clean cloth to remove any pad goop.

If there was no squeal last summer the alignment should be fine. The way to check is to look at the gaps between the pas an rotors. Check from a couple of different angles, the pad should be parallel to the rotor from all angles. If it's not try squeezing the lever - some pads pull slightly off angle when they come off the disk.

If all else fails buy new pads, clean the rotors then install the new pads. Remember that the new pads will need a little running in.

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Check the pads. You just might have some oil or moisture there. Cleaning them with a bit of rubbing alcohol or a specific disc brake cleaner might clear it up for you. Clean the rotors, too. If that doesn't help, you can try replacing the pads, but in my experience if your pads are clean and you're still getting noise from disc brakes, your caliper might need realignment.

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Do you mean caliper needing alignment? Realigning a rotor would be tricky. –  Мסž Apr 2 '11 at 5:38
    
Oops. Yes. I will edit. –  zacechola Apr 2 '11 at 18:41
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When I had similar problem, I rode to the dealers of my bike and they told me that the reason is my pads. Before, I changed the wheel and made any mistake when I set it back. Workers have eliminated this trouble very quickly and cheaply

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If you do shortish rides 20-40 mins grab a spray bottle and spray the brake rotors very lightly tho. It should stop for that period of time if it doesn't stop then it means your brakes are contaminated.

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Spray the rotors with? –  Batman Mar 30 at 13:24
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