The brakes are the most important part of your bike. Failing brakes are extremely dangerous and there are many possibilities that could cause this problem. So make sure your brakes are in good working order by following a simple check over of your bike.
Check the brake cables regularly for fraying. This will normally occur at points of high tension and where the cable is bolted. Lubricating the cable will reduce the chances of fraying. Be careful to never over tighten the cable bolt. This will cause it to fray and break.
Periodically check the state of wear on your brake pads. Pads that have worn down to the metal base will damage the rims. Release the link wire and inspect the surface of each pad. Remove the lip, which normally develops with wear on brake pads used with cantilevers. The lip prevents the cantilever from releasing properly causing the brake pads to drag on the rim.
As the pads wear, you must adjust the position of the brake blocks. The pads should sit firmly on the rim of the wheels, but because the cantilever brake moves through an arc, the pads tend to slip off the rim and into the spokes as they wear down. It is important that the springs on the cantilevers are on equal settings, as they are individually sprung. Most brakes will have an adjustment screw for fine-tuning the spring power.
The link wire is what pulls the cantilever wire together. They will not pull properly if they are kinked. They should be tight through the anchor bolt. The endcap should be attached to the end of a cut wire.
Make sure you lubricate the barrel at the end of the cables. This is the area that brakes are most likely to fail. The best way to prevent the brakes from jamming is to use thick water repellant grease on the cable barrels. Access this by reaching underneath the brake lever body.
You should routinely remove the brake calipers and grease the cantilever studs. When removing a caliper, hold the complete mechanism together to keep the parts in the right order. Note which of the 3 settings the brake spring is slotted into. Once cleaned and re-greased, use Loctite on the mounting bolt to prevent it from rattling loose.
Keep the brake cable housing clean and free of kinks. Get rid of any dirt by removing the inner cable from the housing and use a spray lubricant with a hose. Then put a thicker, Teflon-based lubricant inside the housing before reinstalling the inner cable.
Clean the braking surface of the rim regularly and rub down the surface of the brake pads with a light grade wet-and-dry paper. This will remove the residue, which can build up, and will improve braking performance of the bike.