Project E46 M3 - Rebuilding Brake Calipers September 20 2023

During our last BMW E46 M3 ZCP brake service, I noticed the front caliper piston dust boots had tears in them. I decided to replace these seals and refresh the brake caliper. I bought the rebuild kit from Ate, and they were inexpensive. It comes with the piston seal that seals the piston to the caliper, and a piston dust boot. While I was in there, I decided to replace a few other parts as well. I just did a light rebuild, as most of the parts don't need replacement, but you can certainly opt to replace all the things if you are so inclined. I also didn't elect to paint or powder coat the caliper, but you certainly can if your calipers need a refresh or if you want a color change. If you go that route, I recommend removing the caliper bracket too and painting or powder coating that as well.

E46 M3 brake components shown after the wheel has been removed.

After getting the car in the air and removing the wheel, this is what I was looking at.

Closeup of the anti-rattle clip and the use of a flathead screwdriver to pry against the tension of the clip.

I removed the anti-rattle clip. I like to pry it aft with a flathead screwdriver (shown) and while still prying aft, pry it outboard with another flathead screwdriver (not shown).

Loosening the brake hose with a wrench.

Loosen, but do not disconnect the brake hose, as it is easier to loosen when the caliper is still mounted. We have aftermarket stainless steel brake hoses. Yours may look slightly different if they are stock. Disconnect any brake pad wear sensors if equipped. The sensors can be pried out of the brake pad with a small flathead screwdriver.

Prying off the dust caps that protect the caliper pins.

Pry off the covers to the caliper guide pins with a flathead screwdriver. The upper one is shown, but there is also a lower one.

Removing the caliper guide pins with a hex-bit socket.

Remove the caliper guide pins. You'll need an allen key or a hex bit socket. The upper one is shown, but you will also need to remove the lower one.

Brake caliper loosened and starting to come off the rotor.

With the guide pins removed, the caliper can be removed. The caliper will still be tight at first, so pull outboard on the caliper for 20 seconds or so, such that the piston retracts a bit. Then pull the caliper aft, along with the brake pads. The brake caliper will still be attached to the brake hose. I like to place a container under the area to catch the inevitable brake fluid spillage. I spun the caliper around and around until the brake hose detached. An alternate method is to disconnect the upper end of the brake hose from the hard line, and then the hose is free to thread out of the caliper.

Brake hose capped with the finger of a glove and a rubber band.

The brake line will drip fluid out until the line is dry. It is important to not let the traction control hydraulic unit run dry, as it requires special equipment to bleed and function test if it has been run dry. I used a fingertip of a nitrile glove and a rubber band to stop the fluid from flowing. Another method is to have a pedal prop hold down the brake pedal in the depressed position.

Brake caliper completely removed from the car.

This is how the brake caliper looks like after it has been removed. Next, I cleaned it with water and a nylon brush so that brake dust wouldn't get everywhere.

Air gun attachment pushed into the bleeder hole of the brake caliper.

I want to stress that the next step is quite dangerous. I used a rubber-tipped air gun attachment hooked up to my air compressor to blow 40 psi air into the hole where the brake hose goes. This will force the piston out of the caliper. This is also basically how an air cannon works. The piston will come flying out and spray brake fluid everywhere. Keep all body parts clear of the path of the piston and wear eye protection. I placed a block of wood and a rag in the caliper to help catch and contain the piston. You also don't want the piston to get scratched against the caliper as it is blasted out. This method may sound sketchy, but it is also the method cited in the gasket kit instructions.

I have also previously used the method of pumping the brake pedal with the brake hose still connected to the caliper, which causes the brake fluid to drive out the piston. This method is a bit messier, you lose more brake fluid, and it can be awkward supporting the caliper and working on it while it is still tethered to the car. The upside is that brake fluid is incompressible, so there is not an explosive pop when the piston finally exits.

Packaging that the new brake seals and boots come in.

This is the seal kit from Ate for 1 front caliper. 

Removing of the square seal of the brake caliper.

Remove the square seal from the caliper. You can use a metal pick, but be careful not to scratch any of the surfaces. I prefer using a bamboo skewer, as it minimizes the risk of leaving a scratch that will leak fluid.

Brake caliper piston with dust boot shown torn.

Turning our attention to the piston, the dust boot has seen better days. This tear in the dust boot was the whole reason I rebuilt this brake caliper. You can simply slide the dust boot off of the piston. Light corrosion inside the hollow of the piston is fairly normal and benign. This surface isn't in contact with the brake fluid.

Brake caliper piston with brake dust embedded.

Here you can see where the tear in the dust boot allowed brake dust to get crusted onto the piston. These pistons didn't clean up that well, so I just got a new set of aftermarket stainless steel pistons. The existing pistons are probably perfectly serviceable, but I like things to be nice if it's possible. If you do want to reuse your pistons, you have to make sure the sealing surfaces have a nice, shiny polished finish. Any cleaning process that leaves scratches is not going to be okay, unless you can get the scratches polished out.

New anti-rattle clips and stainless steel caliper pistons next to the old components that are going in the trash.

These are the new parts we are replacing - new pistons and anti-rattle clips. You can replace more or fewer items at your discretion.

New stainless steel brake piston with dust boot in position to go in the caliper.

The new piston is ready to be pushed into the brake caliper. But first, we have to fit the new dust boot. Place the dust boot around the piston as shown, and then get the dust boot seated into the brake caliper.

Brake pad spreader tool pushing against a piece of bar stock that is pushing against the brake piston.

Use a brake pad spreader (shown) or a c-clamp to push the piston into the caliper. I used a scrap piece of metal on top of the piston hollow to give something for the brake pad spreader to push against. I also put a thin layer of assembly lube on the seals and the piston. You want to make sure the piston goes in straight and without excessive resistance. If the piston starts going in crooked or starts to build resistance, don't proceed. Use compressed air to drive out the piston (using caution and proper safety) and try again.

Grease packet and the caliper guide pins that need the grease.

Reassembly is the reverse of disassembly. I like to put some fresh grease on the guide pins. The torque spec on these guide pins is 22 foot-pounds. A word of caution on reinstalling the brake hose: it can be a little tricky to spin the caliper around to thread it onto the brake hose. Do not force anything or it can cross thread. If this method proves challenging, you can disconnect the brake hose from the hard line and then thread the brake hose into the caliper. I was able to spin the caliper onto the brake hose without too much difficulty.

After reinstalling your brake calipers, you'll need to bleed the air out of the brake system. This can be accomplished with a pressure bleeder, or with the two-person brake-pumping method. I prefer the two-person method better, because it moves fluid through the system much faster, which helps to push out the air bubbles more efficiently.

Attach clear pvc tubing to the bleed nipple and then route the other end of the tubing to a waste receptacle that will collect the brake fluid. For a waste receptacle, I like to drill a hole in the lid of an empty glass jar and stick the tubing through the hole. 3/8" inner diameter tubing did the trick for our E46 M3 ZCP.

Before beginning, check the brake fluid reservoir and top it off if necessary. Bleed the calipers in sequence, starting with the caliper furthest away from the brake master cylinder. The bleed nipple can get rounded off easily, so I recommend using a quality flare nut wrench or box end wrench. Open the caliper bleed nipple about a quarter-turn, have an assistant press and hold the brake pedal all the way down, close the bleed nipple and then have the assistant release the brake pedal. Repeat this cycle until the caliper is sufficiently bled. You will see fluid travel through the tubing each time the brake pedal is depressed. You will be able to see bubbles in the fluid for the first few pumps. You are done when the fluid runs clean and free of air bubbles. Periodically check the brake fluid reservoir level after every 10 pumps or so and don't let the level drop below the low mark. Top up the brake fluid reservoir after you are done bleeding the brakes.

You're done with your brake rebuild. Do a function check and verify that the brakes work before driving normally.