Rebuilding Front Calipers
Here's the procedure for rebuilding the front calipers on an E36 M3. The procedure for the rears is almost identical, so it's not worth a separate write-up.
NOTE: Brake fluid will ruin your paint. Keep all brake fluid away from painted parts of the car!
1. Jack up the front end of the car and put it on jack stands. (There are lots of available FAQs on how to do this.)
2. Remove the wheel.
3. Remove the brake fluid reservoir cap and lay a piece of plastic film (like cellophane) over the opening and reinstall the cap. This will help create a vacuum on the fluid so that it doesn't run out the brake lines as fast. Other methods include attaching a vacuum pump to the fluid reservoir or clamping the brake line (for rubber lines only).
4. Detach the brake pad sensor wire (left front and right rear calipers). Loosen the brake line from the caliper just a little (keep it a little snug). This will make it easier to remove in later steps.
5. Remove the caliper spring clip on the outside of the caliper by prying it off with a screwdriver.
6. Remove the two black plastic covers from the caliper guide bolts on the back side of the caliper.
7. Using a 7 mm hex key or hex socket, remove the two caliper guide bolts.
8. Pull the caliper off the rotor. (There is no need to press the piston back into the caliper, since you will be removing the piston.) Pull the brake pads out of the caliper.
9. Remove the brake line from the caliper and plug the end of the brake line with a rubber vacuum cap to keep the fluid from draining out. (Place a small dish under the caliper and brake line to catch any leaking brake fluid). To keep the brake line from twisting, you may need to rotate the entire caliper as you loosen the brake line.
10. Place a piece of wood (about 1/2-1/3 inch thick) between the caliper piston and the outer flange of the caliper. Insert an air nozzle into the brake line hole and using low pressure compressed air, force the piston out of the caliper. (Be careful! The piston will pop out with a lot of force. This is why the piece of wood is there. Keep your fingers away from the piston!)
11. Remove the piston and clean it in brake fluid. Use a Scotchbrite pad to clean any crud off the piston. Inspect the outer face of the piston for corrosion or pitting. If you find either of these, the entire caliper must be replaced -- you cannot buy the piston separately.
12. Remove the outer dust cover (if it hasn't already fallen off).
13. Remove the inner rubber O-ring. It sits in a groove inside the caliper's piston housing.
14. Clean the inside of the caliper piston housing using a Scotchbrite pad, an old toothbrush, and brake cleaner. Make sure you thoroughly clean all the grooves inside the caliper. This a good time to clean the entire caliper with brake fluid (and maybe paint it). Check the inside of the caliper for corrosion and pitting. Let the caliper and piston air dry.
15. Open the caliper rebuild kit. You'll find two parts: an internal sealing O-ring and an outer dust boot.
16. Coat the inner O-ring with either caliper grease (this is a special grease just for calipers - available at most auto parts stores) or brake fluid. Coat the inside groove of the caliper (where the O-ring will sit) with the same. Install the internal O-ring by pressing it into place with your fingers.
17. This next step is the hardest part of the whole procedure. Coat the outside of the piston with either caliper grease or brake fluid. Pull the rubber dust cover over the INSIDE edge of the caliper, with the lip of the dust cover (the lip that will be inserted in the caliper, not the piston) facing away from the piston. The edge of the dust cover that will eventually be seated in the groove on the piston should be facing the piston groove, and the rest of the dust cover should be hanging slightly over the edge of the piston. While holding the piston near the caliper housing, push the lip of the dust cover into it's groove in the caliper. Then gently push the piston into the caliper. The piston will hit the inner O-ring and become more difficult to press in. Keeping the piston exactly straight, gently tap it into place using a hammer against a flat piece of metal or wood held against the piston. You can also insert the handle end of a small hammer into the piston and then push the piston into the caliper.
18. You can skip this next step, but it will make it easier to bleed the caliper because it primes it with brake fluid. Loosen the caliper bleed screw and using either a vacuum on the bleed screw or pressure on the brake line hole, fill the caliper with brake fluid. (I use a Mity-Vac pump to pull fluid into the caliper.)
19. Reattach the brake line, insert the brake pads into the caliper, and install the caliper on the rotor. (Don't tighten the brake line yet - just a little snug.) Again, it may be necessary to rotate the entire caliper as you tighten the brake line in order to keep the brake line from twisting.
20. Reinstall the caliper guide bolts (torque to 24 ft lbs.), plastic covers, and spring clip. Then tighten the brake line. Reattach the brake pad sensor wire (left front and right rear calipers).
21. Remove the cellophane, vacuum, clamps, etc. and bleed the caliper (there are plenty of other FAQs on how to do this).
22. Remount the wheel and lower the car off the jack stands. Remember to pump the brake pedal a few times to get the pads reseated against the rotor before you go for a test drive.