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Replacing Mercedes Benz Rear Rotors and Calipers
 
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Pelican Technical Article:

Replacing Mercedes Benz Rear Rotors and Calipers

Steve Vernon

Time:

5 hours5 hrs

Tab:

$110

Talent:

***

Tools:

17mm socket, rubber mallet, T30 Torx, zip-tie,

Applicable Models:

Mercedes-Benz W123 (1977-85)
Mercedes-Benz W124 (1986-95)
Mercedes-Benz W126 (1981-91)
Mercedes-Benz W201 (1984-93)

Parts Required:

Brake discs, new pads, new emergency brake shoes (if required)

Hot Tip:

Adjust your emergency brake while you have access

Performance Gain:

Better, safer braking

Complementary Modification:

Replace brake pads, emergency brake shoes, install stainless steel brake lines, or install new wheel bearings

Brake discs (or rotors as they are often called) are a very important part of the braking system. The brake pads rub against the discs to create a friction force that is responsible for slowing the car down. If the rotors become too thin or develop grooves in them, then their ability to stop the car decreases.

When replacing your brake pads, you should always measure the thickness of your brake discs. If they fall below the specified value for your car, then they should be replaced with new ones. Check for grooves in the rotor, and make sure that you take several measurements of the disc in several different places. This will guarantee that you get an accurate reading. If the brake disc has a groove in it, then it should most certainly be removed and resurfaced by a machine shop or simply replaced with a new one. Discs with grooves not only brake less efficiently, but they also heat up to higher temperatures and reduce your overall braking ability.

The measurements that you take with your micrometer should be made from the center of the disc. It is common for OEM rotors to have the minimum thickness stamped on the rotor hub or check your owner's manual.

If you do find that you need to replace your rotors, the process is a relatively simple one. The procedure for the front or the rear rotors is very similar, but for the sake of this project, we'll look at replacing the rears, which is slightly more complicated due to the addition of the rear parking/emergency brake. With the rear rotors, if the parking brake shoes are worn, then you may need to back off the adjustment sprocket in order to be able to remove the rear disc.

If your calipers are leaking, damaged or need to be rebuilt, this article will take you through the steps of replacing them as well.

The first step is to jack up the car and remove the wheels. Next remove the pads from the calipers. The pads are held within the caliper by two retaining pins. There are also small retaining clips that hold these two retaining pins in the caliper. Start by tapping out the retaining pins using a small screwdriver and a hammer. When the two retaining pins are removed, the anti-rattle spring that holds the pads in place will fall out. Now the pads can be pried out with a screwdriver. In high corrosion areas the caliper may need to be dismounted to hammer the pads out, then file the caliper body/pad side contact area to make new pad installation possible. Use the small holes on the pads that normally surround the retaining pin as a leverage point for removing them. They may require some wiggling to remove, as it is sometimes a tight fit. It is important to keep in mind that the caliper piston is also probably pressing against the pads slightly and will add to the difficulty in removing them.

Now, unbolt the caliper from the hub where it is mounted. There are two 17mm bolts that mount the caliper and hold it in place. After you remove these two bolts, you should be able to move the caliper out of the way of the disc. Exercise caution when moving the caliper around--make sure that you do not let the caliper hang from the rubber brake line, as this will most certainly damage the line. You are gong to be disconnecting the brake line from the old caliper and if you are not replacing it, attaching it to the new caliper later, for now leave the line attached and hang the caliper by a zip tie or rope out of the way. Inspect your lines for any damage and replace as needed. I also recommend replacing the lines if they have not been replaced in the last eight years.

Once you have the caliper out of the way, remove the small screw that holds on the brake disc with a T30 Torx. At this point, make sure that the parking brake is off. You should now be able to pull the disc off of the hub. If there is any resistance, use a rubber mallet to tap the brake disc off. Sometimes the disc will require some heavy smacks with your rubber mallet to get it off.

If you are having a difficult time getting the disc off, it's probably because the parking brake shoes are stuck on the back of the disc. You might need to adjust the parking brake so that it's not gripping the disc. Anything except the lightest corrosion on the hub face must be cleaned with scotch brite and/or 80 grit sanding discs to keep the mating surface true.

Installation of the new brake disc is a snap; simply push it onto the hub. Before you install the new disc, take a close look at your parking brake shoes and see if they warrant replacing. If you can see metal on the shoes, or if the previous owner had a hard time remembering to release the parking brake, then it might be a good time to replace these. After you install the new discs on both sides, you should test your parking brake and adjust it if necessary.

After the new disc is installed, replace the retaining screw, reattach the caliper, and install new brake pads.

You are now going to change over the brake lines. You will need to bleed the system after this, so while there will be a little spillage you should be prepared for, you really want to make sure no dirt, debris of foreign matter gets into the new calipers.

New calipers usually arrive with the pistons already fully pressed in, check them and carefully press them in (I use an old brake pad and C-clamp) if they are not. Remove the rubber grommet from the top of the new caliper. While being careful not to get anything into the opening, use a 14mm flared wrench and remove the line from the old caliper, and attach it to the new one.

When you have installed all four corners, you are now ready to bleed the system.

Please see our article on bleeding your brakes. DO NOT drive the car until you have bled the brakes!

Your new rotors should last a long time, and you should see an improvement in your braking after the wear-in period for your new brake pads.

To remove the old pads, tap out the retaining pins (green arrow) with a screwdriver or punch and a small hammer.
Figure 1

To remove the old pads, tap out the retaining pins (green arrow) with a screwdriver or punch and a small hammer. They should slide out pretty easily, as there is usually no load on them. If there is much difficulty encountered during the removal process, then tap on the pads slightly to remove pressure from the pins.

Pulling out the pads usually involves the use of a screwdriver for leverage.
Figure 2

Pulling out the pads usually involves the use of a screwdriver for leverage. The pads are loose in the caliper, but it's a pretty tight fit, and there is usually lots of dust and debris in the caliper. Wiggle the pads back and forth in order to pry them free. Although these parts usually can be reused, some people prefer to install new retainer kits. The kits include two new retainer springs, and four pins that are used to hold the pads into the caliper.

You do not need to push the piston (yellow arrow) back any more than enough to get the pad out.
Figure 3

You do not need to push the piston (yellow arrow) back any more than enough to get the pad out.

Remove the two 17mm bolts holding the caliper to the knuckle (yellow arrows).
Figure 4

Remove the two 17mm bolts holding the caliper to the knuckle (yellow arrows).Hang the caliper from a zip-tie or rope so that you don't put unnecessary tension on the rubber brake line.

There is a small locator screw that holds the brake disc in place.
Figure 5

There is a small locator screw that holds the brake disc in place. Use a T30 Torx tool to remove this screw, make sure the parking brake is not on, and the brake disc should slide off of the hub. Keep in mind that the lug nuts that hold on the wheel apply the majority of the force that constrains the disc to the hub--not this screw.

The new disc can be tapped on with a rubber mallet.
Figure 6

The new disc can be tapped on with a rubber mallet. If installing the rear discs, make sure that you have your parking brake shoes adjusted away from the inside drum, or they might interfere with the installation of the disc. The red arrow shows the adjustment mechanism for the brake pads. If you are having trouble removing the rotor from the hub, you can access this with a screw driver through where the lug goes and turn it back to release pressure on the parking brake pads (yellow arrow). New discs may not be perfectly flat and may take a few hundred miles of break-in to achieve their maximum braking efficiency. You can see the old caliper suspended by a zip tie (green arrow), so it is not putting pressure on the brake line.

This photo shows the new rotor or disc installed with the locator screw on (green arrow), and the new caliper and pads attached.
Figure 7

This photo shows the new rotor or disc installed with the locator screw on (green arrow), and the new caliper and pads attached. All that needs to be done now is remove the rubber grommet from the new caliper and install the brake line. You want to be very careful that you do not get dirt of debris into the new caliper while doing this. Remove the old caliper from its holding place (yellow arrow) and using a 14mm flared wrench remove the line (red arrow) from the old caliper and install on the new. You now MUST bleed your brakes. Do not attempt to drive the car until you have bled the brakes.

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