Why are my brakes squealing?
This is most often due to pollution. Sometimes this is a bad setting, sometimes it is a warning about the need for new pads, sometimes it is because the pads are polished, or from the wrong material, but more often it is dirt. Keep all oils, degreasers and bike cleaners away from pads and discs to prevent squealing. And, of course, the brakes must be worn in.
A deformed rotor causes friction in the brake every time the wheel is turned.
Why do I sometimes hear brake friction when not braking?
Disc brakes operate to very close tolerances, usually just a millimeter or less of space between the rotor and the pads. This leaves little room for error. If you occasionally hear brake friction, this is either a sign of a bent disc, or the brakes need service, or an external influence. Friction can be caused by the flexibility of the hubs and frame (forks). Do you sometimes hear these sounds when you tilt your bike while cornering or sprinting? If so, that's flexibility. First, check if your axles are tight. Also make sure the hubs don't play. If all this does not solve the problem, make sure that the caliper is correctly positioned with even pad clearance on both sides of the rotor. If this does not correct the situation, then the problem may simply be in the flex axis or the installation of dropouts. Sometimes the pads rub because the pistons behind them stick. This can be due to accumulated dirt, and sometimes simply to prolonged use. Servicing the brakes (with reinstalling and cleaning the pistons) and bleeding them usually solves the problem. Finally, there may be enough dirt, sand, or just water to make the discs shuffle or even howl. If you just drove through a puddle and the discs rub a little, do not worry about it. The very first press on the brake is likely to clear the dirt and restore silence.
My brakes rub, but only at one point on the wheel. What to do?
It looks like the disc is deformed. It is only about two millimeters thick, so it may well bend. Be careful not to lean your bike discs against anything and be doubly careful when transporting your bike in a car or case so as not to bend them.
If this happened immediately after a long descent, it may be due to overheating. Allow the rotor to cool down, and if it is still deformed, follow the advice below.
If the rotors are not critically bent, then they can be pulled out or bent into their original shape. This is usually done with the wheel mounted on the bike, using the brake pads as an indicator. Take a piece of clean paper towel to touch the surface of the disc, and gently press down with your thumb at the rubbing point to bend the disc backwards. Take it easy! It doesn't take much effort to bend the rotor back.
For greater accuracy, there is a special tool (Disc Brake Calliper Alignment Tool). Alternatively, an adjustable wrench can be used, but make sure it is perfectly clean before using it.
My hydraulic brakes are rubbing constantly. What to do?
You just shot the wheel and now they rub like crazy? If so, see the section above on traveling with disks. It talks about how to put the brake pads (and pistons) back into the caliper in order to reinstall the system.
If this was not the case, your brake caliper probably needs to be aligned. Look through the caliper to see if the rotor is rubbing on only one side. Yes? Okay, then you need to loosen the two bolts holding the caliper in place. Most calipers can then be automatically aligned by pulling on the brake lever and pressing the pads against the disc. Then, while holding the lever, tighten both bolts. Sometimes it doesn't help, but there are many other tricks to deal with it.
Align the caliper visually and tighten it gently (with a white sheet on the ground to improve visibility).
Another option is to use a business card between the rotor and the block that rubs. With the card in place, repeat the process above. Loosen the caliper bolts, press the lever and tighten the bolts while holding the lever. This may take several tries, but it usually solves the problem if the brakes do not want to automatically snap into place.
If you've just installed new pads and the clearance is minimal, try pushing the pistons back into the caliper bores (use a plastic tire raiser or the rounded back end of a small wrench). Sometimes there is too much fluid in your brake system (this happens if the brakes were quickly bled or the system was replenished with worn gaskets. You need to bleed the system again or, carefully opening it, release a little fluid (without letting air!).
Several people have tried to adjust my brakes, but no matter what, the pads do not properly hit the braking surface of the rotor. What to do?
If the caliper alignment is correct and your rotor is straight, then it looks like the frame (fork) is to blame. Ask your frame manufacturer how to fix the problem, but they will most likely suggest a replacement (preferably at their expense, of course).
Sometimes paint or simply poor manufacturing tolerances can cause caliper alignment problems. Park Tool offers a tool called DT-5.2 Disc Tab Facing. This tool removes material until the brake caliper mounting surface is parallel to the axis. This is an extremely expensive tool, so look for a workshop that has it (unfortunately, there aren't many of them, but ProVelo does have one).
My disc brakes are running out of power. What can be done?
This could be due to several reasons, but it is most likely one of the problems below.
First, it is most likely that the braking surface is dirty. Even a little greasy sweat from your fingers can reduce the effectiveness of disc brakes. Cleaning the system can help, but if it is heavily soiled, new pads and a thorough rotor cleaning are needed.
On extremely long downhill runs, lining may occur. This can be determined by their glassy surface. Remove the pads and sand them with sandpaper - that should do the trick.
Air in the system can also be to blame, which interferes with the efficient transfer of power from the lever to the caliper. Bleeding the brakes will solve this problem (see the section on bleeding the brakes above).
Finally, it may be that your brake rotors are not large enough for your riding style or body weight, especially if your bike is now equipped with 140mm discs. Find out if larger rotors will fit on your bike and provide more leverage.
What if the hydraulic brake lever falls very close to the handlebars?
It looks like there is not enough fluid in the system. Check to see if bleeding is required by following the guidelines in How Do I Know When My Brakes Need Bleeding? (section above).
If this is not the case, then it is known as leverage failure. You have several options.
Pumping the brake pads closer to the rotor will provide less lever movement. To do this, remove the wheel (and rotor) and carefully press the lever until you see the movement of the shoes. Reinstall the wheel and check the feel of the lever. Repeat this procedure until the lever moves properly. Keep in mind that this will make it difficult to fit without friction and will only work as long as the pads are not worn out.
Some brakes also have a lever travel adjustment. If you have one, then there is an adjusting screw that you can use to reduce its travel.
Why do my brakes lose power when braking for a long time?
Sometimes this is a sign that the brake fluid is overheating and starting to boil. When this happens, which was previously incompressible, the fluid becomes a compressible gas and the braking quickly loses power. In this case, it is highly recommended to upgrade to a larger rotor size or install heat dissipating pads and discs (such as Shimano's Ice Tech).
It is also possible that there is air in your system (see the section above on leeding the brakes).
What if the brakes suddenly lose power?
Well, this is bad!
The first thing to do is try to pump up the brake. Just press the lever quickly for some time. If it is air causing sudden brake problems, then pumping the lever will likely move the bubble from the hose to the upper reservoir.
If this does not work, then most likely you will only have a service brake on one wheel. Use it carefully to get to the house, and then contact a mechanic for service (repair). Although very rare, a faulty component could be a possible culprit for a system failure.