If your car is pulling to one side when braking, then there is a good chance that you might have a sticky caliper that needs rebuilding. The rebuilding process is actually a lot simpler than most people think. It basically involves removing the caliper, cleaning it, and then reinstalling all of the components with new seals. Very often, the most difficult part of the task is the process of actually removing the caliper from the car.
The first step is to jack up the car and remove the caliper. Refer to Project 55 for details on removing the caliper from around the brake disc. Refer to Project 54 for more details on disconnecting the brake line from the caliper.
Once you have the caliper free and clear from the car, take it over to your workbench and begin the disassembly process.
The first step is to remove each of the four pistons from the calipers. One method of removal is to use compressed air to blow out the piston. Using a small screwdriver, remove the dust boot that surrounds the piston. Place a small block of wood in the center of the caliper to prevent the pistons from flying out of the caliper. Blow compressed air through the caliper bleeder hole to force the piston out of its chamber. Start slowly and gradually increase pressure until the piston reaches the block of wood. Make sure that the piston doesn't come all the way out of its chamber. After the piston is far enough out, you should be able to get a grip on it with your fingers. Be careful when working with the compressed air, as it is more powerful than it appears and can make the pistons suddenly fly out of the caliper unexpectedly.
Using a rag to protect the sides of each piston, carefully remove all of them from the caliper using either your hands or a large pair of vise grips. Make sure that you don't touch the sides of the pistons with any metal tools, as you don't want to scratch this surface.
If the piston is frozen, then more radical methods of removal may be necessary. Using a block of wood, you can try pounding the half of the caliper on the block of wood until the piston begins to fall out. If the piston starts to come out and then gets stuck, push it back in all the way and try again. Eventually, the piston should come out of the caliper half. Another method is to use the car's brake system to release the pistons. Reconnect the caliper to the car and have an assistant pump the brakes to force out the piston.
Once the pistons have been removed from the caliper, carefully clean both the inside and outside of the caliper using brake cleaner or another appropriate solvent. All of the passages should be blown out with compressed air, and it's a good idea to let the whole assembly sit in some parts cleaner overnight. If the piston or the inside of the caliper is badly corroded or pitted, then the caliper should be replaced. A little bit of surface rust is okay--this should be polished off using a coarse cloth or some Scotch-Brite. Make sure that you thoroughly scrub out the entire inside of the caliper and the piston so that they are perfectly clean.
After the caliper and piston have been cleaned and are dry, coat the caliper and piston with silicone assembly lube. If you don't have this silicone assembly lube handy (it's available from PelicanParts.com), then make sure you coat the entire assembly with clean brake fluid. Do not get any lube or brake fluid on the dust boot.
Insert the new piston seal into the inside of the caliper piston groove. It should fit smoothly in the groove, yet stick out only slightly. Make sure that you wet the seal with a little brake fluid prior to installation. Now install the dust boot onto the piston, so that the edge of the boot fits into the inside groove of the piston. Then, insert the piston slightly into the caliper. It should slide in easily--make sure that it doesn't get cocked. Push the piston all the way into the caliper. As the piston reaches the internal O-ring, you will encounter some resistance. Make sure that the piston doesn't become cocked as you insert it. If you have trouble inserting the piston into the caliper, you may want to softly tap it with a plastic hammer or use a small piece of wood to compress the piston into its home position in the caliper housing. Finally, make sure the outer rim of the seal mates properly with its groove in the caliper housing.
Install the brake pads (Pelican Technical Article: Replacing Porsche 911 Carrera Brake Pads), and remount the caliper back onto the car. Bleed the brake system (Pelican Technical Article: Bleeding Porsche 911 Carrera Brakes), and you should be good to go. Make sure that you carefully check the brakes on the car before you do any significant driving.
Remove the outer seal by simply prying it out of the caliper housing. The new seals are installed in reverse fashion, but may require some finesse work to get them properly seated in their grooves.
The best method I've found for removing the caliper pistons is to use compressed air to blow them out. Beware though--start with low pressure and then increase it if it's not enough. It's easier than you might think to send your pistons flying across the room with 100 psi of air pressure!
Make sure that you replace the inner piston seal. This seal is what keeps brake fluid from leaking out past the cylinder. Also be sure that you clean the entire inner cylinder for dirt, debris, and corrosion. Don't scratch the inside of the caliper cylinder while you are working on it or you may have problems with the caliper leaking when you reassemble it.