Brake units usually last for years without complaint. Generally, the first thing to go will be the cover that protects and contains the spring. Spares for these are not generally available, but without the cover, the spring splays open under pressure rather than snapping the units briskly away from the rim. Replace units with broken spring covers; they cost barely more than a new set of brake blocks, which come supplied with the new units.
There is not as much choice available in cantilever brakes as there used to be once upon a time. There are however still some very goodquality models are still made for cyclocross bikes; for example, Avid Shorties are light, strong and powerful, though some people say they squeal too much.
Step 1 : Pull the quick-release on your brake unit down and out to release it from the slot on the unit. If the slot in the brake unit has become squashed, use a thin screwdriver to carefully ease the slot apart so that the cable slides out without damage.
Step 2 : Cut off the cable end, undo the cable clamp bolt and release the cable from the units. Use this opportunity to check the condition of the cable, particularly where it gets clamped under the cable clamp bolt.
Step 3 : Remove the bolts at the bottom of each unit and pull them off the frame. You may need to twist and pull at the same time; dirt that works its way into the gap between the brake and pivot can stick the two parts solidly together. For stubborn cases, try spraying a light oil into the gap, then work the brake back and forth.
“If your brake pivots are dirty and corroded, your brakes will feel sluggish”
Cleaning the pivots
Clean the pivots on the frame. If they’re lumpy, uneven or gritted, use a scrap of wet-and-dry sandpaper. Oil the pivots and slide the new units on to them. Line up the spring that protrudes from the back of each with the middle of the three holes on the frame beside the pivot bolts. The new bolts may come with a stripe of Blue Loctite 242, but if not, it’s worth adding one since the last thing you need is your brakes rattling loose. A thin stripe will do, 2mm (1⁄8 inch) wide or so, for most of the length of the bolt.
Refit the cable, setting it up so that the two sections of link wire or straddle wire are at 90 degrees to each other. You may need to loosen the eye bolts that hold the brake block in place and push the brake blocks back away from the rim to give yourself enough space. Once you have set up the cable, follow the instructions for fitting new brake blocks.
If your brake pivots are dirty and corroded, your brakes will feel sluggish. Clean and oil pivots regularly for crisp braking.
Undo the quick-release and the cable clamp bolt and pull the cable out completely. Undo the fixing bolts and remove them completely. Pull the units off the frame and clean them. Use wetand dry sandpaper to clean the pivots. Undo one of the eye bolt nuts, remove the eye bolts and clean all the curved washers. Put a drop of oil on each of the mating surfaces and reassemble. Just do one at a time so that you can refer to the others to put the unit back together in the right order. Oil the brake pivots and slide the brake over the pivot, lining up the spring on the unit with the middle of the three holes beside the pivot on the frame. Refit the bolts that hold the units on, then follow the instructions for fitting new brake blocks.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES