Project E36 M3 - Rebuilding Brake Calipers September 24 2013

During our squealing brake project, we noticed that the rubber dust boot on one of our brake calipers had deteriorated. Since this can allow moisture into the brake system, which can cause corrosion, we decided to replace all the seals before problems could occur. We set out to take everything in the caliper apart, clean everything up, respray some paint, put in some new seals and then put everything back together. It's not that difficult of a job. So let's get started.

To begin, get the corner of the car safely in the air and remove the wheel.

Remove the rattle clip by pushing it aft with a screwdriver and then popping it outboard.

There are two bolts that hold the brake caliper on. Undo these with a 7mm allen wrench or allen socket. 

Loosen the brake line and then snug it back up. This will allow us to undo the brake line more easily later when the caliper is hanging free.

Remove the caliper from the rotor. This usually requires pulling outboard on the caliper for a good 20 count and then wiggling the caliper aft off of the rotor. Remove the brake pad wear sensor and the brake pads. Hang the caliper from a rope or stiff wire so that the brake line isn't bearing the weight of the caliper. Now put a container under the caliper to catch the brake fluid that is going to pour out of the caliper. Cardboard under the area is a good idea, since it will soak up spills and also protect the piston from hitting the concrete if it falls out. Now go inside the car and pump the brake pedal. The pressure will drive the piston out of the caliper. 

We also removed the caliper bracket because we wanted to clean it up and respray it, but this is typically not required.

Here are all the parts laid out. Now we're going to clean everything up and put it back together.

Remove the piston seal from the caliper. I like to use a bamboo skewer to dig it out because a screwdriver has a chance of scratching the surface. Also remove the bushings that the attaching bolts go through. We have upgraded bronze bushings that are held in with snap rings. The OEM setup is simply a rubber bushing that can be pushed out. We removed the bleed screw too.

Now we turn our attention to cleaning everything up. Our right hand side piston had some moderate corrosion on it that wouldn't clean up with polishing compound, so we went one step more aggressive and cleaned it up with some green Scotchbrite.

After a good 30 minutes of scrubbing it, most of the corrosion cleaned up. It isn't perfect, but it's certainly better than it was before.

The caliper bore was mostly clean, but had some light corrosion. Some polishing compound and a Dremel cloth wheel did the trick.

We cleaned the parts using brake cleaner, a pressure washer, and a steel wire brush. If we were to do it again, we'd skip out on the pressure washer, since the water caused some flash rust to form on the inner bore of the caliper. We removed the flash rust with some polishing compound. After everything was cleaned and degreased, we masked all the surfaces that we didn't want paint on - the bore, the bushing holes, the bleed screw hole and the brake line hole. We laid tape over the areas in question and cut the excess with a razor blade. We chose a high temperature paint specifically made for brake calipers.

Following the paint manufacturer's instructions, we did 2 light coats and then a heavy final coat, waiting 10 minutes between coats.

With the masking tape removed, this caliper is starting to look good.

24 hours after the paint has been applied, it is supposed to be cured at 200 degrees F for an hour. We tried hitting it with a heat gun for a while, but weren't able to get it hot enough.

I took a random metal box that I had and put the parts inside. 

There was a slit in the front, which we used to blast hot air into. We managed to get the parts up to 300 degrees F and then left them there for a while.

Now it is time to put in the new seals. We coated the seals and all the mating surfaces liberally with assembly lube. On my first time around, we didn't use enough lube, and it had a hard time going in. 

We put the inner piston seal on and then we slid the rubber dust boot onto the end of the piston. We let the dust boot hang off the inboard end of the piston so that we could work it into the caliper groove first before sliding the piston in.

Push the piston in by hand until it won't go any farther.

Use a C-clamp or a disc brake pad spreader to push the piston in the rest of the way. It should go in without much resistance. If you meet resistance, pull the piston back out and try again. Our piston got stuck on the first try, and we had to hook it back up to the car, load up more brake fluid, and pump the brakes to push the piston back out. On our second try, we used more lube and a disc brake pad spreader instead. We were met with success.

Put the bleed screw back in, but be certain not to over-torque it. Holding the wrench on with my thumb, I use my middle finger to torque it so that I can't apply too much torque. Put the bushings back in, and screw the caliper back onto the brake line. Mount the caliper and brake pads back onto the car and then tighten up the brake line. Bleed the brake system, and you're good to go. Our caliper looks brand spankin' new and the seals should be good for another couple of decades. We were also able to catch some corrosion before it got out of hand.

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