Brake Caliper rebuild - PeachParts Mercedes-Benz Forum

chomptown · #1 04-30-2010, 01:12 PM

Hey everyone, I've been fixing up an '83 240D for my wife to use as a daily driver. I was getting down to the brakes, as they were squealing a bit, and I could feel a bit of drag. At this point I must say that Haynes manuals have caused me nothing but misery in the past, and once again I find this to be the truth. The manual says that you should pull the brake wear sensor before pulling the pins. Well, after tugging with needle nose pliers(as per the manual) I hear a nice *snap*. I immediately stopped pulling, but I definitely cracked the plastic of the brake wear sensor. I eventually saw that it was being held in by the anti-rattle springs... The sensors just popped out when I removed the pins and pulled out the springs. Now here's my first question.

One of the brake wear sensors is missing the black covering on the end where it attaches near the brake pad. I'm not sure if it was pulled out when I removed it, or if it just wore away. Will this affect anything? This is also the same one that had the nice *snap* when I pulled on it. I'm not sure how these brake sensors even work. Does the bare wire make contact with the rotor, and that is what sets off the sensor? Any help would be appreciated. (I miss mechanical brake wear warnings...)

Now my second question. One of the rubber boot covers for the piston has a good 1/2" hole in it. Should I just rebuild the whole caliper? My brother, along with others I've spoken to, said to just stick the new pads on and forget about it since it's just the dust boot. I'm worried that it would allow foreign material to get near the piston and rust it. Let me know what you all think. Also, if there's anything I should be watching out for while doing the rebuild, please let me know. Thanks

Great forum by the way! It's helped me out quite a bit so far.

rhodes2010 · #2 04-30-2010, 01:33 PM

First the sensors:
Contact with the rotor wears off the head of the sensor and as you said
a ground signal then sets off the light.

The sensors are cheap and considered disposable and should
be replaced when pads are replaced if they have set off the light.

I don't replace mine or use them anymore, I am pretty good at
knowing how long the brakes will last.

As for the caliper, if it works properly you probably do not need to
worry about rebuilding it. Yes, dust boots do deteriorate but should be okay.

What part of the country are you in ? Maybe your weather conditions
would be a cause for concern.

Others here will have better input on the caliper.

chomptown · #3 04-30-2010, 01:42 PM

Thanks for the quick reply. I'm in Portland, OR. We have a ton of rain, but we do not salt our roads.

Here's another quick question. In the past, when I've replaced pads on a car/truck, it seems like the rotor can spin freely. I was not getting any free spinning with this rotor when the pads were on. Do you get a full rotation if you spin your rotors by hand?

Diesel911 · #4 04-30-2010, 02:19 PM

Gunk in the Caliper bore and a stiff Calper Seal can prevent the Capliper Piston/s from retracting properly.
While it is not unusual for the Pads to drag just a touch it should take no effort to turn the Wheel with the Tire on and the Tire off of the ground.
Another give away is if one side turns easier than the other.

Diesel911 · #5 04-30-2010, 02:45 PM

Below is a pic of how the Caliper Piston Seal pulls the piston away from the Disc. When the Piston is moved inwards the Seal distorts. When you release your Brakes there is no hydraulic pressure and the Seal returns back to its shape and retracts you Caliper Piston a little.
So if your Caliper Piston Seal is stiff and loses elasticity it cannot pull your Caliper Piston Back.
Even if your Caliper Piston Seal is good it dose not have a lot of force to pull it back. Gunk under the between the Piston (I think this is most often the cause) and the Caliper wall can prevent the seal from pulling the Piston Back

If you for some reason let the Brake Fluid level get too low and some air got into the system and you later refilled the fluid. An Air bubble will also keep a Caliper Piston from retracting.

chomptown · #6 04-30-2010, 02:49 PM

Is there a dust shield on top of the piston also? On some of the rebuild kits I've seen, it shows there is a dust shield or something.... Would this keep back some of the gunk that could get past the damaged boot? I would prefer to hold off on rebuilding the calipers right now. But I don't want to put my wife in any danger.

Edit: Thanks Diesel911, that diagram explains it well.

rrgrassi · #7 04-30-2010, 03:00 PM

If money is not an issue, just replace the calipers and the rubber brake lines. It does cost more, but saves tons of labor related work. If it was my car, my wife would be upsetthat I did not do it right, safety wise. I also would not send her on the road with a potentially ready to blow caliper.

Where I live, I do not encounter salted roads, but I did have a caliper sieze up on me a month ago. The dust boot was torn, and we did have lots of rain. Rust made it sieze up. The smell of hot brakes and a hot rear wheel confirmed the siezed caliper. It was the inboard piston that siezed.

chomptown · #8 04-30-2010, 03:16 PM

Money is definitely an issue for me. I'm a cheap-ass, and I would prefer to pay $11 for a rebuild kit, rather than the $75 reman'd caliper. This might be one of the few times my wife will appreciate me being so anal. It looks like I'm going to have a fun weekend.

rrgrassi · #9 04-30-2010, 03:29 PM

Make sure you note the position of the pistons. They have a slight notch out that needs to be positioned back into the proper orientation...I learned that on the rear caliper rebuild I did. I totally understand the money thing.

I advise you to rebuild both calipers and replace the rubber lines.

chomptown · #10 04-30-2010, 03:55 PM

Thanks rrgrassi. I'll drain the fluid and take off both front calipers. The system probably needed to be bled anyways, since there is most likely rust in the fluid too. If it all seems to be going well I might take some pictures to document it.

Thanks for the help guys. If anyone has any input or tips, please feel free to chime in. I'm doing the removal tonight, and will try to rebuild them this weekend.

chomptown · #11 04-30-2010, 04:29 PM

So apparently what I thought was a dust shield is actually some sort of metal bracket that looks like it shields the rubber dust boot from being damaged by the pin. Its the one in the center, which my vehicle currently does not have. Am I correct in assuming this?

tangofox007 · #12 04-30-2010, 04:58 PM

Quote:

Originally Posted by chomptown

So apparently what I thought was a dust shield is actually some sort of metal bracket that looks like it shields the rubber dust boot from being damaged by the pin.

Heat shield.

Mesalm · #13 04-30-2010, 06:00 PM

Yeah I second HEAT SHEILD.

Diesel911 · #14 05-01-2010, 12:06 AM

Quote:

Originally Posted by chomptown

Is there a dust shield on top of the piston also? On some of the rebuild kits I've seen, it shows there is a dust shield or something.... Would this keep back some of the gunk that could get past the damaged boot? I would prefer to hold off on rebuilding the calipers right now. But I don't want to put my wife in any danger.

Edit: Thanks Diesel911, that diagram explains it well.

The Pistons have to be installed at a angle. However, as you can see in the other Pic the Heat Shields, at least on the ATE Brakes (that is a front Caliper) kind of show you what that angle is. (Mark the original position of the Pistons before removing them anyway.)

By the way I was not speaking of gunk that gets past the boot but the mix of Water and Rust that end at the bottom inside of your Brake Calipers. Caused by the fact that Regular Brake Fluid absorbs Water from the Air and they continue to make calipers out of metal that will rust. At least the plate the Caliper Pistons.

chomptown · #15 05-01-2010, 01:36 AM

Oh ya, that makes sense. I was wondering why they all seemed tilted in a bit. My right front caliper was in perfect condition, so hurray for that. I took the pistons out and kept the heat shields on so that I know how they should be aligned. I have the bendix calipers, but they seem very similar. I didn't find any rust so I imagine I could have gotten by without rebuilding this one. It would have ended up needing to be done eventually though.