This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
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 a 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 47 for details on removing the caliper from around the brake disc. Refer to Project 48 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. You don't have to disassemble the two halves of the caliper if they weren't leaking, however, if you fully want to clean out the caliper, then you should take them apart. Keep in mind that sometimes it's difficult to reassemble them without having them leak, so be forewarned. Most caliper kits also do not contain the o-rings that seal the two halves to each other.
Now, you can remove the pistons from the calipers. One method of removal, suggested by Bob Tindel of Pelican Parts is to use compressed air to blow out the pistons. Using a small screwdriver, remove the dust boots that surround the piston. You may need to remove the retaining ring in order to remove the boots from the caliper. Place a small block of wood in the center of the caliper to prevent the pistons from falling out of the caliper. Then, take a c-clamp and clamp down one of the pistons. 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 that you can get a grip on it, remove the c-clamp and place it on the other side so that you are keeping the already extended piston from traveling out any further. Blow compressed air through the bleeder hole again, and the other piston should move out of the other half. 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 the pistons, carefully remove them both 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 pistons are frozen, then more radical methods of removal may be necessary. Disassemble the calipers into their two halves. Then, using a block of wood, pound the half of the caliper on the block of wood until the inside 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 inner pistons. Make sure that one piston doesn't fall out of the caliper, or you will have difficulty removing the other one.
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 rusted or pitted, then the caliper should be replaced. A little bit of surface rust is ok: 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 pistons so that they are perfectly clean.
After the caliper and pistons have been cleaned and are dry, coat the caliper and piston with silicone assembly lube. If you can't find this silicone assembly lube at your local auto parts store, 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. Now, insert the caliper into the piston. It should slide in easily: make sure that it doesn't get cocked. On the top of the piston, there is a portion of the groove that is recessed. It is important to align this recess at a 20 degree angle to the brake pads. Refer to the diagram accompanying this project for reference. This angle is required to keep the brake pads from squealing when pressed against the brake disk.
After you make sure that the piston angle is correct, push the piston into the caliper half. Make sure that the piston angle is correct when you have the piston pushed back into the housing. If it is not correct, then you need to pull the piston out again (compressed air, etc.) and realign it.
When both pistons are installed, fit the dust boot around the pistons and attach the retaining clip. On the rear calipers, it helps if the pistons are sticking out about a 1/4 of an inch from the housing in order to get the boot to fit.
Once the dust boots are installed, remount the caliper back onto the car. Install the brake pads (Pelican Technical Article: Replacing Brake Pads), bleed the brake system (Pelican Technical Article: Bleeding 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.
Rebuilding calipers is a lot easier than you would normally 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 so that they return to their original gold color. Rebuilding your calipers may solve a lot of mysterious brake problems that you may have been experiencing. Because the process of rebuilding seems difficult, it's usually the last project tackled when overhauling the brakes.
Make sure that you replace the inner piston seal. This seal is what keeps brake fluid from leaking out past the cylinder. Make sure that you clean the entire inner cylinder for dirt, debris and corrosion. Be careful not to 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.
This diagram shows the proper orientation of the caliper pistons inside the caliper. Make sure that the notches on the caliper are offset at the 20-degree angle before you reattach the caliper to the car. This angle is important because the pistons are designed to reduce brake squeal when installed in this orientation. It's easy to make your own template by photo copying the diagram on this page.