How do I replace the brake pads?
It's a good idea to give a general cleaning to the calipers every now and again with brake cleaner and a rag (be sure to keep brake cleaner spray away from the rubber boots). While you are installing new pads is a good time to go beyond routine cleaning, and to check and lube all the parts of your caliper. In fact, installing new pads with a sufficiently dirty caliper is a guaranteed trip to the bike shop for new seals... Maybe not today, maybe not tomorrow, but guaranteed indeed.
Make sure you don't have the brake pads in place while you're cleaning, though. The best practice is to keep every fluid possible off the pads. Air and water are okay, but anything else can lead to pads that get slick, get hard (providing less friction) or even get crumbly and wear away in just a couple of uses. It all depends on how the glue that's binding the particles (metal or organic) together reacts to whatever chemical hits it.
Caution: As with all things to do with braking, be thorough and careful. And only do one caliper (front or rear) at a time. Take a test ride and make sure the brake you worked on works correctly before you take the other one apart. That way you know you have at least one functioning brake.
When to replace
The service limit for pads is 1mm. There should a wear indicator on the pads, but it won't be there if you're past the service limit. On stock pads there is a little "step" that shows where the service limit is. You should probably think about new pads before you get too close to 1mm.
The front pads shown below fell victim to that most infamous of scoundrels, Mr Previous Owner. These went to 14,000 miles, though they should have been replaced about 1000 miles before. They have about 0.25mm of pad left, which is well past the 1mm service limit.
Doing the work
To start, remove the two bolts holding the caliper to the bike, and remove the caliper. This is pretty straightforward, although our man with the camera here decided to do some extra work while he was in the vicinity. He has removed the bracket which the torque link attaches to (and which the rear axle passes through) (outlined, picture left). This is not necessary in order to replace the brake pads. You only need to remove the two allen bolts circled in red (picture right). You may need to pivot the muffler can out of the way in order to access the top bolt.
The front is pretty easy to figure out. Remove the two 12mm bolts attaching the caliper to the fork.
Gently remove the little brake pad and set it aside.
Pull back (compress) the caliper bracket, thus pushing the caliper guide pins in as far as possible.
Shift and tilt the big brake pad so it looks like this.
Push one edge of the brake pad over the caliper pin, allowing you to easily remove the other side shortly thereafter.
Your caliper should now look like this.
Remove the anti-vibration bracket (take note of its orientation).
Clean everything which is now visible. NO dirt/water/oil whatsoever is ever allowed on the brake's inner parts: the outer surface of the piston or the caliper's fluid chamber. Dirt on the inside (cup-side) of the piston is fine, but you should try to spray it out anyhow, to keep the brake piston insulator from seizing inside the piston (cup). It's best to get it all as clean as possible now.
One part of the pistons that is nearly impossible to clean is the underside, the bottom of the piston just above the caliper body. Just spraying the pistons with brake cleaner usually isn't enough. There is really no way you're going to get a brush between them and the caliper bottom, either.
The solution is to rotate the pistons, so you have access to that area. This is simple, but you must be VERY CAREFUL to not scratch or score them, or leave tool marks. If you do, you will have to buy new ones.
In order to do this, it is necessary to understand the design of the pistons. A brake caliper piston looks like nothing more than a large, smooth thimble, or a wee little cup. The bottom of the cup goes into the depth of the fluid chamber, and inside the cup sits a black plastic insulator. This insulator is what actually touches the pads. Removing the insulator (it will fall out easily unless your caliper is really nasty) exposes the inside and outside of the cup, leaving a lip you can grip to rotate the piston.
You can't just go grabbing away with a pair of pliers, though. The technique involves removing the plastic covering (where the “+” is) from the pistons. When the piston is out of the caliper (ie... before you push them back inside) the lip of it is exposed. You then want to put a scrap of leather between your pliers and the piston to protect it. This will enable you to turn the piston into a cleanable place without harming it.
Under normal use, do not top up your brake fluid unless you're below the low-water mark (which is unlikely unless you have a leak), or you have brand new pads on. Why not? Because if you do, the next time you change your pads and push the pistons up the caliper, you will have to open your master reservoir and let some fluid out. Be careful when pushing the pistons back into the caliper; if you have too much brake fluid in the reservoir, it could overflow. Not only would you have a mess to clean up, but brake fluid eats paint. Quickly.
Push the pistons back in. If they're clean, you should be able to do this with your thumbs (push HARD). This should take some effort, but not a lot. The key here is CLEAN pistons. If you have removed the brake cylinder reservoir cap since the last time you changed your pads, you may also need to remove this cap to push the pistons in.
If for some reason you can't use your thumbs, use a 6" C-clamp bearing on an old brake pad to push the pistons back in. This should be considered a backup method only. Note that you'll need to push both pistons at the same time, or pushing one will cause the other to pop out.
Here the caliper has been cleaned with brake cleaner and a nylon detailing brush (giant toothbrush) and the pistons pushed back inside.
Note that the pistons must be ABSOLUTELY clean, and the outsides of them absolutely smooth, before being pushed back inside the caliper. Also check that they show no signs of pitting or scoring. If one or both has more than light pitting, it is due for replacement, as it can chew away at your dust/fluid seals and cause your brakes to gradually lose fluid. Even small amounts of pitting on the piston edge can be a problem, "wicking" fluid past the fluid seals, etc...
Removing the pistons (optional)
If you're really thorough, you can remove the piston entirely to clean the fluid chamber, but then you'll need to bleed the brakes when you're done. The fluid chamber can accumulate dark sediment, particularly on older or abused bikes, and it's best to remove that. Be careful to not damage the piston or caliper while you are doing this; aluminum is soft.
If you have compressed air to blow into your caliper and dislodge the piston, great. Wrap it all up in a towel first, so your piston doesn't come shooting out and damage anything. This also keeps brake fluid mist from blowing around your work area. If you don't have compressed air, you can also blow the piston out with a bicycle pump, head held tight against the caliper line opening.
You also can get the piston out by simply removing the brake pads and, keeping the caliper hooked up to the undrained system, pump your lever/footpedal. The fluid will push the piston out. Cut off the top of a plastic jug and put the whole assembly in it, to keep mess to a minimum.
Wear nitrile gloves to keep that brake fluid off your skin, and keep plenty of rags around to wipe off excess fluid. Brake jobs can get messy.
The caliper is wide-open in this shot; the large-pad carrier has been pushed all the way back on the slider pins. Note the position/shape of the boots, they are accordionned-closed and bulging slightly with trapped lubricant.
This is a good time to inspect the seals around the pistons. If there is a leak, or a seal is torn/dried out, it is time for a rebuild. See Brake caliper rebuild.
To show you the location, look at the rear brake diagram below. The dust seal is #43049. So long as it's there, and it's not obviously messed up, it's fine. The fluid seal, #43049A, is inside the caliper, behind the piston. The front brake has different part numbers.
Reinstall the anti-vibration spring. The anti-vibration spring reduces pad vibration, and helps to hold the small pad in when the caliper is off the rotor. The small pad can still fall out, though. No major tragedy; just put it back in.
Put the small pad in first. You must push down slightly against the anti-vibration spring to slide the pad into position. The corners of the pad fit into cut-outs in the caliper which are the same shape.
The small brake pad has been installed.
Open the caliper by holding the caliper as shown and compressing. The back plate should slide back on its pins.
Hook one side of the large brake pad over its post.
Rotate the brake pad around, while keeping the caliper wide open. Make sure the left side is all the way "out", so that the pad stays straight when slipping it over the right-hand pin.
Now the brake pads are back in the caliper. Don't bolt it back up yet: always inspect the sliding pins on the caliper first.
Lubricate the caliper pin. Add silicone-based brake lube underneath the two boots to keep the caliper pins moving freely.
Make sure the boots are not nicked or torn, and are making a good seal with the caliper. The boots' job is to trap lubricant next to the pin, and to prevent it from being washed away by rain.
Reinstall the caliper. Torque for the mounting bolts is 24 ft-lbs. front and 18 ft-lbs. rear.
Pump the brake lever until it gets hard. If you removed the pistons, you'll need to bleed the brakes, which is covered in this article. A hard brake lever should be achievable in less than a dozen strokes.
Test the brakes while walking before going for a ride. It's more pleasant to realize something's not working when you're walking next to it than when you're approaching a stop at a busy intersection.
Follow the bed-in procedure for your new pads.