This article is one in a series that have been released in conjunction with Wayne's new book, 101 Performance Projects for Your BMW 3 Series. The book contains 272 pages of full color projects detailing everything from performance mods to timing the camshafts. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any 3 Series owner's collection. The book was released in August 2006, and is available for ordering now. See The Official Book Website for more details.
If your car is pulling to one side when braking, you might have a sticky caliper that needs rebuilding. The rebuilding process is actually a lot simpler than most people think. Rebuilding involves removing the caliper, cleaning it, and then reinstalling all of the components, along with new seals. Removing the caliper is often the most difficult part of the task.
First, jack up the car (see our article on jacking up your BMW). Then refer to Project 57 for details on removing the caliper from around the brake disc. Moving on, Project 56 will guide you in 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.
Start by removing the piston from the calipers. 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. Then 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 the piston doesn't come all the way out of its chamber. After the piston is far enough out, you should be able to grip it with your fingers. Be careful when working with compressed air, as it is more powerful than it appears and can make the pistons fly out of the caliper unexpectedly.
Using a rag to protect the sides of the pistons, carefully remove them both from the caliper with your hands or a large pair of Vise-Grips. Don't touch the sides of the pistons with any metal tools, which may scratch their surfaces.
If the piston is frozen, more radical methods of removal may be necessary. Using a block of wood, pound the caliper half 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 piston has been removed from the caliper, carefully clean both the inside and outside of the caliper with brake cleaner or another appropriate solvent. Blow out all of the passages with compressed air. If possible, let the whole assembly sit in parts cleaner overnight. If the piston or its cylinder is badly rusted or pitted, replace the caliper. A little bit of surface rust is okay but should be polished off with a coarse cloth or Scotch-Brite. Thoroughly scrub out the inside of the cylinder and outside of the piston so they are perfectly clean.
After the caliper and piston have been cleaned and dried, coat the cylinder and piston with silicone assembly lube. If you don't have this silicone assembly lube handy (available from PelicanParts.com), coat the entire assembly with clean brake fluid. Be careful not to get any lube or brake fluid on the dust boot.
Insert the new piston seal inside the caliper piston groove. It should fit smoothly in the groove, yet stick out slightly. Wet the seal with a little brake fluid. Now install the dust boot inside-out onto the piston, so the edge of the boot hangs out toward the area where the piston contacts the brake pad. Then, insert the piston slightly into the caliper. It should slide in easily, but make sure it doesn't go in crooked. Wrap the dust boot around the outer edge of the caliper, and then push the piston into the caliper.
As the piston reaches the internal O-ring, you will encounter some resistance. Make sure the piston is pushed in and doesn't become cocked as you insert it. If you have trouble inserting the piston into the caliper, 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.
Now install the brake pads (Project 51) and remount the caliper onto the car. Bleed the brake system (Project 50), and you should be good to go. Of course, carefully test the brakes on the car before you do any significant driving.
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Rebuilding calipers is a lot easier than you might think. The basic principle involves tearing apart the caliper, cleaning it, and then reinstalling the pistons with new seals and clips. Professionally rebuilt calipers, like this one, are usually sandblasted and plated so they return to their original gold color. Rebuilding your calipers may solve a lot of mysterious brake problems you've been experiencing. Because rebuilding seems difficult, it's usually the last project tackled when overhauling the brakes. Another option is to purchase rebuilt calipers and mount them on your car.
Replace the inner piston seal, which keeps brake fluid from leaking out past the cylinder. Also, clean the inside of the cylinder of dirt, debris, and corrosion. Don't scratch the inside of the caliper cylinder while you are working on it, or the caliper may leak when you reassemble it.