Inspect your brake blocks frequently for wear, replacing them as they get thin and pick up grit and swarf. Worn blocks make for a useless braking surface and eat expensive rims for breakfast. Ignore this important task and your rims will wear right through the rubber of the block to the metal innards of the moulded pad. Brake blocks have a wear line indicating when they should be changed. If you can’t see a wear line, change the pads when they’ve worn to the bottom of the grooves moulded into the pad.
Even if you don’t wear out blocks very quickly, you should still change them periodically as they harden with time and don’t work as well. Every couple of years should do.
In between full changes, check on the condition of the pad and improve it. This is easiest to do with the wheels removed.
Release the brakes and remove the wheel. Look at the condition of the pad. It should be flat and even, without visible contamination. If you can see flecks of metal, use a sharp knife and carefully pick them out. If the pad has been sitting too low or at an angle, it will wear unevenly, leaving a lip that gets caught under the rim and prevents the brakes from letting go properly. This is a waste of brake block and braking potential. Carefully cut the offending lip off with a sharp knife, then follow the instructions to reposition your brake block so
that it contacts the rim more evenly. If the brake block sits too high, it will wear through the tyre – an expensive error. Lightly sand the surface of the brake block with clean sandpaper. People often use a file for this, but shouldn’t – it will hold metal flakes from whatever it was you last filed, and they will now embed themselves in the blocks. Clean your rims too. If they have sticky black streaks, use degreaser. Oil or tar on your rims will squeal alarmingly, allowing your wheels to slip through the blocks without slowing you down. A green nylon washing-up scourer works well for stubborn stains, and will scrub off contamination without damaging your rims.
Some brake blocks are designed with removable rubber blocks. The old, worn ones are removed by pulling out a retaining pin at the back of the metal cartridge and sliding the rubber part backwards. Replacement rubber blocks slide in in the same way and are held in place with the retaining pin. Always make sure the open end of the cartridge faces towards the back of the bike; otherwise heavy braking will rip the rubber out of the cartridge. The replacement blocks can be stiff to slide into the slots in the cartridge; it often helps to dip them in warm water.
Step 1 : Loosen the brake block fixing nut. Wiggle the block so that its top edge doesn’t touch the tyre, and the bottom edge doesn’t undercut the rim. The top and bottom of the block should be equal distances from the rim, with the front of the block 1mm closer than the back – ’toe-in’. Once it’s right, tighten the fixing nut firmly – hold as shown so the block doesn’t twist as you tighten.
Step 2 : Adjust the cable tension: undo the cable clamp bolt, holding the cable as shown so you can use your other fingers to steady the brake unit. Pull or release cable so that the gap between brake blocks and rim is around 2mm. Retighten the cable clamp bolt firmly, then spin the wheel. The rims should pass freely between the brake blocks, then lock when the lever is pulled halfway to the bars.
Step 3 : Fine-tune the brake using the barrel adjuster at the brake lever. It’s normally prevented from working loose by a lockring turn this anticlockwise several turns to release the barrel adjuster, then roll the barrel adjuster anticlockwise (A) to move the brake blocks closer to the rim, or clockwise (B) for more clearance. Spin the wheel, retest and readjust. When you’re satisfied, roll the lockring clockwise to wedge it against the brake lever.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES