Calliper brake blocks don’t wear out as quickly as V-brake blocks, but they’ll still need occasional attention.
The calliper mechanism means you can’t apply as much force to the block to stop you as you can with a V-brake, so it’s worth investing in good-quality brake blocks and replacing them when they’re worn. Good makes include Aztec, Shimano and Kool-Stop. Of the three types of rim brake, calliper blocks are the easiest to change. Give your rims a good clean at the same time, and check them for wear. There are only two types of fitting for brake pads most common are 4 or 5mm Allen keys, but some need a 10mm Allen key. You may decide when you’ve got the old brake block off that it will survive a little longer – if so, clean it up carefully. Use a sharp knife to pick out any bits of metal or glass, then flatten the surface with sandpaper. Refit in the same way as a new block.
Step 1 : Start by removing the old brake block. Undo the bolt at the back and slide the block downwards and outwards. Inspect the surface of the block. You may decide that it will survive a little longer, in which case follow the instructions above to clean it up and then refit as below.
Step 2 : If you’ve got the kind of brake blocks that have replaceable pads, undo the screw on the back of the pad. The old pad should then slide backwards out of the brake shoe. Clean up the shoes then slide the new pad in and replace the screw. It is very important to replace the brake blocks the right way round: the opening on the block must face towards the back of the bike.
Step 3 : Check for marks indicating the direction the new blocks must face. The block may be marked ‘left’ and ‘right’ or be curved, in which case the shape of the curve must match the shape of the rim. The block may come supplied with shaped washers. If so, fit the cup-shaped washer so that the flat side rests against the brake flock. Fit the domed washer so that the curve of the dome faces into the cup of the washer you just fitted.
Step 4 : Wriggle the brake block into place. It should be parallel with the rim, but not so high that it will touch the tyre nor so low that any part of it would hang down below the rim. The curvature of the fitting washers will allow you to make minor corrections to the angle. Once you’ve got it in place, hold the block still with one hand while you tighten it with the Allen key, or spanner.
Step 5 : Next, check the toe-in. Pull the brakes and watch the gap between the rim and the brake block. The front of the block should touch the rim just before the back – the difference should be about 1mm 1⁄25 in). If it’s not correct, loosen the fixing bolt slightly, adjust the angle of approach and tighten the fixing bolt firmly. Check that the brake block is at the right height as you pull and release the brake-lever.
Step 6 : The new brake blocks will probably be a different thickness to the old ones, so you will need to adjust the clearance between the brake block and the rim. It should only be a minor adjustment, so use the barrel-adjuster on the calliper. Loosen the lockring slightly to get it out of the way, then adjust the clearance using the main part of the barrel-adjuster. Retighten the lockring.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES