Brake pads must be changed if they’ve worn down to less than a third of their original thickness, so replace them if you only have 0.5mm or less of compound left on the backing pad. You will also need to replace them if they have become contaminated. This could be the result of brake fluid leak, careless lubrication of the drivechain or other environmental factors.
Bedding in brand new brake pads is vital. If you skip this task you’ll find that you suffer both poor braking and vanishing disc pads. New pads need to be ’cooked’ to remove chemical residues left over from manufacture and harden the braking surface.
You do this by braking to heat up the pad, then allowing it to cool, and repeating until the braking performance improves, so find yourself somewhere that you can ride safely without having particularly sharp brakes (ie not a busy A-road or a steep descent). Then ride along slowly, before braking until you’ve nearly come to halt. Repeat this process until the braking performance improves; it’s not unusual for it to take ten to twenty cycles. Be aware that the braking performance will improve suddenly so try not to go over the bars! Once the pads are well bedded in, they usually last a long time. Exactly how long will depend on the trail conditions: gritty mud will wear them out faster than soft, sloppy mud, whilst dry trails could easily see you riding the same trails all summer long, though if you’re running sintered pads you may find that they glaze and require a quick scuff with sandpaper to improve the bite and reduce the noise. Pad fitting is similar enough from brake to brake for it to be possible to just give a general overview here. It’s worth having a good look at your calliper before you start taking anything apart, even taking a picture or drawing a quick sketch if you’re worried you might forget what it should look like.
Unfortunately almost every brake model from every manufacturer requires a different shape brake pad (there are exceptions – Shimano’s newer systems all use the same pad shape). Take the old pad to the bike shop if you’re unsure of the brand and model names of your brake to make sure you get the right replacements.
Removing the old pads
As a general rule, you need to remove the wheel to get at the pads and they fit into position via the central slot in the calliper, either from the same direction as the rotor or from the top. Each pad sits against its piston inside the calliper. When you pull the brake lever (or master cylinder), each piston (or slave cyinder) is pushed out, taking the pad with it until it contacts and slows the rotor. Pads have a flat metal backing plate, onto which the compound is bonded. The backing plate will usually have a metal tab which allows you to handle it without needing to touch the braking compound. Pads can be left- or right- specific and though this information is sometimes stamped/etched on the backing plate, it’s wise to make a note of which way round the old ones go.
Step 1: The pads will be held in place by a screw, a magnet, a spring clip on the back of the pad or simply a very precise fit in the calliper. There is often some sort of secondary security device to stop the pad falling out if the first fix should fail, so remove this first.
Step 2: Then remove the main retaining device, in this case a small Allen key bolt.
Step 3: Now the pads are free, you can remove them – simply pull gently on the tabs and they should come out with little more than a wiggle. If you’re not planning on putting the same pads back into the calliper then use them to protect the pistons as you push them back into the calliper body, before you remove them.
Disc brake callipers self-adjust as the pads wear out. The pistons will protrude slightly further as the overall pad width shrinks and this can make it very hard to fit the new pads with their full thickness compound. The vast majority of brakes are now open systems, so you will need to very gently push the pistons back into the calliper once you have removed the old pads. You can use a tyre lever if you are worried about damaging the piston – they are relatively fragile, particularly those with a small locating pin that’s easy to snap off. You may find that you need the leverage supplied by a longer tool though, so use a large, flat blade screwdriver wrapped in thick cardboard or a ring spanner which sits neatly on the face of each piston. Push one piston gently back into the calliper body until it sits flush, then repeat with the opposite piston. You may find that as you push in one piston, the other moves out again; persevere with a few more cycles but if the pistons refuse to sit evenly and flush with the calliper, you will need to bleed them.
Step 1: If dirt is allowed to lodge behind the pads, it can potentially damage the piston seals, contaminate the brake fluid and cause airleaks. Before fitting new pads give the calliper body a good clean. Use a clean rag or tissue, with plain soapy water – there should be no need to use a degreaser. Avoid solvents and brake fluid, too, as these can damage the silicon piston seals.
Step 2: Most pads use a spring to spread the two faces apart when not being pressed to the rotor. Fit this to the new pads, making sure you’ve got both left and right pads the right way round. Avoiding touching the braking compound, press the two pads together and make sure the spring is neatly fitted. Ensure that the spring arms sit beside, not over, the face of the brake pad.
Step 3: Push the pair of new pads into the slot in the calliper, reversing the action you took to remove the old pair. Push them in until they are firmly in the right place within the calliper and any retaining pin on the piston lines up with the pad’s locating holes. Then fit the retaining pin, and finally the secondary retaining device, so that the brake looks exactly as it was when you started. Refit the wheel and pump the brake lever once or twice to settle the pistons into place. Finally, bed in your pads.
A word about bite
Many modern brakes will have a bite point adjuster. This is usually a thumbwheel or a screw located by the lever. Screwing this in and out alters the internal geometry of the master piston in the brake lever, and consequently moves the pads in and out of the calliper. This allows you to control how far the lever moves before the pads hit the rim. Sometimes this can also have an effect on the lever reach as well so you may need to reset your reach adjuster after you have finished. This feature is useful as it allows you to set the bite point of your brake to where you want it, something that was not previously possible on open system brakes. When bleeding a brake that has a bite point adjuster it is important to set it all the way back before you start, so that the slave pistons in the calliper are retracted as far as possible. This will give you the full range of adjustment once you’ve finished bleeding the brake.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES