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Old 11-20-2014, 03:55 PM
Lucas's Avatar
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Join Date: Sep 2014
Location: Santa Barbara, CA
Posts: 1,202
Caliper rebuild and brake job

Upon removing my caliper for a bearing job, I noticed my brake pad wear was uneven. As well, the rotor had a lot more grooves on one side, and clean on the other.

I was already planning on rebuilding the caliper as preventative maintenance, once it seizes you are looking at a difference of $10 as opposed to over $100. Plus the heat stress on all those parts.

However, the uneven pads points to a problem.

I haven't seen any caliper rebuild write ups here yet, so I figured I should share. There two types of calipers on these car, read the stamp on yours to verify which kit to order. I had ATE, but this write up should apply to both. I ordered from the parts store and got lucky, they didn't ask.

There are three main components to each piston. The seal for the piston inside the bore, the boot that drops into a groove on the piston, and a clamp for the boot where it attaches to the caliper.

What ends up happening is the boot on the piston gets dry and loose, and let's moisture in. There is a seal slightly inside the bore for the piston, and the area between the edge of the bore and seal gets corroded. The piston hangs up and becomes sticky, and this causes uneven pad and rotor wear.

Eventually moisture gets to the seal, and past it, and your caliper freezes. All hell brakes (pun intended) loose. The piston freezes out and can't go back it, heats your rotor red hot, cooks you bearings, and you cry at replacement part costs that could have been prevented.

The easiest procedure for removing the Pistons out of the caliper is compressed air. Shoot your compressor into the bolt hole for the brake line, and one should pop out. Place it back in slightly with a c-clamp holding it, or cover the hole with an old brake pad and c-clamp, and the other should pop out with more air.

Do this in a drain pan and wear safety goggles (not glasses). Brake fluid will get everywhere.

Mine were corroded between the edge of the bore and the seal, still salvageable but slightly frozen. Air did not do the trick. The most effective and messiest solution is to place a bolt in the brake line hole, and inject grease through the bleeder with a grease gun.

You can try penetrating oil and soaking for days, I'm impatient.

An easier method to try first is to use the car. Wire the caliper up and connect the brake line. I got mine without bleeding, fortunately.

A buddy pressed on the brakes until the piston started to move. Stop before it comes all the way out, but go as far as possible.

Place a c-clamp on the piston to hold it in position. Medium sized ok, mine's a little overkill.

Press the brakes until the other side comes out. You can let it come out all the way out. Fluid will come out too.

Now the tricky part. Some people do one side at a time to avoid this, but I like to sandblast the caliper in the cabinet and don't want the piston in there.

If you are lucky you can wiggle the first piston out. If not, cover the open bore with an old pad or the piston slightly in place and a c-clamp. If air doesn't work, you will have to use the car. Place the removed piston back in slightly past the seal and clamp in. You can clean up the bore to avoid it sticking after the other is removed, or chance em.

Many times, I've gone back and forth sticking left and right. Remain calm.

My second piston came out with the pad and clamp covering the opposing bore and a shot of air.

Now look for grooves or cracks in the piston or bore. If any are present, you're done. Go buy calipers. A little rust is ok, but extreme corrosion will reduce the size of the piston or bore. I have actually never seen this yet.

Now we focus on cleanliness. Cleaning and painting is not just to look pretty. Grease holds moisture and corrodes things.

I covered the bore of the caliper with the old pads and small c-clamps and sandblasted it. It had a thick enamel paint on it that was hard to remove. If you don't have a blasting cabinet, or access, you can use degreaser and a wire brush. A little old paint is ok.

After blasting I removed the pads and cleaned around the bore with a wire brush.

I also remove the backing plate and and cleaned/painted as well. Backing plates are underrated as far as what they do, keeping moisture and dirt out. Removing requires dissembling the parking brake, which emphasizes how important I feel it is.

Take a look at that dirty, water retaining device.

Sandblast or wire brush it with degreaser, just focus on the grease.

After both caliper and backing plate are cleaned up, time for paint. I like to use an engine enamel in a spray can, as these parts can get warm if you are out hot roding. Tape over the bores in the caliper, and try to cover the outside edge where the boots sit. My tape wasn't sticky enough, so I sanded that outside edge after to remove the paint.

Have a beer and let it dry. Place the backing plate back on. Someone did this before me, Mercedes or a good mechanic, but I always silicone the inside edge to the hub flange. They also siliconed the hole for the parking brake springs, so I learned something something new.

I added a 1/8" hole to the very bottom lip where the bend is, to allow a drain. Whenever I'm under the car I hit it with compressed air. With any car, think about where moisture may retain and add holes. Mercedes has a great design here with that inner ring opening up at the bottom. It's those details that make us love these cars.

Toyota trucks for example, usually have holes, but they clog up. I make them larger and blow them out often.

Check out how clean it looks now (pre-hole, sorry about that).

Time to put your caliper back together. This can be a challenge, but with multiple piston brakes it tends to be easier. The boot can be a real fight with a big large single piston, smaller is better.

First clean up the piston and bores. I wire brush the Pistons first, then use sand paper on them and the bore. They say 1000 grit, these were bad and I touched the outside portion with 220 first, never beyond the sealing edge with under 1000. You don't want any scratches or grooves.

Get down in the lip for the boot (in the piston). And the lip for the boot on the caliper as well. You need a clean surface for the boot to seal.

Clean/sand the edge where the piston contacts the pad. I noticed mine had 1/4 of this edge indented, probably to allow moisture to escape, and I made sure to orient it to the ground.

Everything must be clean. Work on a clean surface and put on clean gloves. Spray the bore and Pistons with brake cleaner and blow out with your compressor. Make sure to clean through the brake line hole as well. Cover each bore so you get the channels through each side.

Put the seal into the groove in the bore. Coat the inside of the bore and and piston with brake fluid for assembly lube (I use my pinky).

The boot/piston order is always different, but this is for rear Mercedes calipers. The brake pad box said about 1960 to 123 chassis.

For this one, it was easy. I pushed the piston all the way in first. I slid the boot over the top, and dropped the small part into the groove on the piston. The larger side of the boot stretched over the lip on the caliper. My kit came from the local store, and included a key ring style clamp for the boot on the caliper side. A little weak but easy to install. Next kit I'll order from a reliable source, I'm sure better quality.

The trick was spreading the ring and placing the two ends against each other. It slid over easy with a little spreading at the end. It pinched the boot slightly and I had to play with it to get it all seated down.

You can attempt to put some fluid in the caliper to avoid a longer bleeding procedure, but with new pads it wasn't bad.

Now I should be good for a LONG time. I had my rotors cut to insure a proper seating of the pads, and will need new ones next time. I'll clean the backing plate every job, but due the hub needing to be removed I won't paint it again until next bearing job, maybe 25 years. Otherwise, I remove and paint the backing plate on a judgement call. On this car it's rare, on a 4x4 truck that crosses rivers, more often.

You can buy calipers, but I enjoy this job and trust myself more than a factory.

I got a little OCD and painted my rotors as well. I did this before having them cut in case I had an accident and got paint on the contact surface.

Step back and admire your work.

Long term build thread:

1970 220D in for the long haul

Last edited by Lucas; 11-21-2014 at 02:32 AM.
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