We find it easiest to work on one brake unit at a time, removing the old block then replacing it straight away with the new one. This way, you will keep the washers and eye bolt in the correct order. If you take both blocks off at once, the washers and eyebolt have a tendency to drop off one unit when you’re working on the other.
Hold the eye bolt still using a 5mm Allen key in the head of the eye bolt, and undo the nut at the back of the unit – usually with a 10mm spanner. You don’t need to take the nut off, just loosen it enough to pull the old brake block out of the hole in the eye bolt. Feed the stub of the new brake block back through the hole in the eye bolt so that the brake block faces the rim. Most brake blocks will fit either way around, but if there are any arrows printed on the block, point these forward. If one end of the brake block is shorter than the other, this goes at the front.
Step 1 : Tighten the nut just enough so that the brake block doesn’t fall out. Don’t worry about adjustment at this stage, as long as the block is pointing vaguely towards the rim. Leave a generous gap between rim and brake block at this stage. Repeat with the other side. Now we’re going to leave the brake blocks for a bit and set the units up at the right angle.
Step 2 : For straddle wire types, adjust the height of the straddle hanger and the length of the straddle wire, so that the two halves of the straddle wire are at 90° to each other. Link wire types are much simpler – undo the cable clamp bolt, pull in or let out until the two arms are at 90° and reclamp. Most link wires are stamped or printed with a helpful guide line.
Step 3 : Next, the balance screw. Cantilevers usually only have one. Pull brake lever and watch the units. If one sits closer to the rim, adjustment is needed. If the unit with balance screw is closer, turn screw clockwise to strengthen spring and move it out. If it is further away, turn balance screw anticlockwise. Start with half-turns, pulling the levers to ease things into place. At first there’s no effect, then the spring gets sensitive to quarter-turns, so move slowly.
Step 4 : Once everything else is set up, return to the brake blocks. Loosen the nut on the back of the unit so that you can manipulate the brake blocks. Push in each one until it’s almost touching the rim. Each block should hit the rim at 90°, midway between the top and bottom of the rim.
Step 5 : The front of the block should be about 1mm (1⁄16 inch) closer than the back. This is called ’toeing in’ and helps to prevent the brakes from squealing. People often mess about with bits of cardboard stuck behind the back of the brake block. This is just making work for yourself – look at the brake block and the rim, and set the angle of the brake block so that it’s closer to the rim at the front than at the back.
Step 6 : Hold the block in place with your hand and tighten the 10mm nut gently. Once it is fairly secure, hold the eye bolt still with a 5mm Allen key and tighten the 10mm nut firmly. Try to waggle the brake block – if it moves, it’s not tight enough. The cable will now need fine-tuning, ’Adjusting the cable tension’.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES