Found a number of threads, but thought I would start a new one!

I have ATE calipers on rear of my 85 300D. They were replaced 16 years ago. While in process of replacing discs, pads and parking brake shoes, I noticed that one disc had worn unevenly and that one piston on that side was sticky. Boot had a small split in it, so water/salt may have entered.

Decided to have a shot at rebuilding myself. Watched a few videos, but new job for me. Ordered FTE repair kits.

I removed one caliper. I clamped one side expecting the other side to stop at the clamp. But it came right out (Using air pressure). So now I will probably have to reinstall this piston, put in a spacer and then push the other one out? Does that sound right?

I have inspected the piston and cylinder that is open. I noticed there is a build up of some yellow stuff on the inner cylinder surface. Light rust outside the seal. Will try and clean up with scotch pad or ?? Wonder what caused that yellow deposit? Piston looks good, but has some light rust build up on outside edge where boot attaches.

Some pics.

Piston


Cylinder

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

I got the other piston out by reinstalling the boot, covering with a piece of 1/8" aluminum plate and clamping. This sealed the open cylinder and air pushed the other piston out.

Again, piston is near perfect, but the cylinder does have that patchy yellow coating. It is even under the main seal. Whatever it is, scotchbrite pad doesn't take it off.

Without splitting the caliper, it is not that easy to access the cylinders for cleaning. Not going to split, so need some good ideas

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Not sure about the patchy yellow stuff you refer to. Is it a corrosion resistant coating?

I rebuilt mine a while back and made one big mistake. I did not replace the pistons. They had some rust/pitting but it was all negative, and since the seal doesn't slide on that surface I figured it was non functional. Well, not 6 months later they were sticking again. I replaced them with reman'd calipers.

I think typically you should use a brake cylinder hone on the ID to even the surface. You might be able to get it shiny again, but scotchbrite will unevenly remove material leaving a low, non-cylindrical spot which might allow fluid to leak by the seal. I don't have a feel for how sensitive they are to that, but I'd wager the spec is within .001". If you can find a spec sheet on the seal manufacturer's website, it might have a mating part tolerance listed.

I doubt it is. It seems to be quite random. Some even under the seal. I suspect caused by some sort of degradation of the brake fluid when hot. Maybe moisture related.

Regarding sealing, the pistons are near perfect and they ride on the square seal. I think that so long as the seal groove is clean, the new seals should seat and seal properly.

I will clean up as best I can. It would be easier if I split the caliper, but don't want to go there. Thinking I might be able to make a hone of some sort. Also maybe soak caliper in brake cleaner or acetone overnight and see if that softens that yellow stuff!

Seal kits not coming until end of week, so have time to clean!

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

When I don't split them, I work the cylinder walls with emory paper and any built up gunk with a razor blade. It is awkward for sure, but doable.
If you split the caliper then its a breeze with a brake cylinder hone, but you need the square orings that hold the calipers together and they don't come with all kits.
I usually polish the pistons themselves with a big leather welding apron I have till they are bright again.
I use a little grease for assembly and to protect from rust in that area between the cylinder oring and the dust boot that always seems to be rusty and crusty.

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1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660

I am not keen on using an abrasive like emery paper. But a razor blade or xacto knife might help on a few small hard spots. Do you know if the cylinders are chromed like the pistons?

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

I wouldn't think so. If you don't like Emory paper, then you wouldn't like the brake hone either, but that sure does clean them up fast.

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1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660

A couple notes after reading thru this:

As Graham mentioned it is the seal groove that is important, not the cylinder bore. The bore must be clean to allow the piston to slide freely but it is the seal groove that the seal works against... and the piston sides of course. The groove must be clean, no pits or deep scratches. The piston sides must be smooth with no pits or deep scratches parallel to the axis.
PMB Performance rebuilds Porsche calipers primarily but also do other makes. They have a good tech page which describes the rebuild process:

https://pmbperformance.com/pages/the-ultimate-brake-caliper-restoration-guide

They are not as familiar with Mercedes calipers as they are Porsche so it would be wise to inspect the calipers post rebuild, if you have them do it, for correct piston orientation and dust seal installation... direct experience here. Their plating process is nice. Direct experience also requires me to say that the plating is important in reducing corrosion that will lock up your calipers again in a relatively short time since they do see water exposure externally and brake fluid is hygroscopic, it absorbs moisture. Many lower end caliper rebuilders that supply the corner auto parts stores do not re-plate and you'll find you have sticky calipers within a short time. Make sure the reservoir cap bellows is in good shape and that you replace the fluid every two years or so to remove the inevitable accumulated moisture.

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Owner of Mobil-Werks
Classic Sports and Specialty Automobile Service
1968 230 W110 +3.5 M116 & 4-Speed Manual
1985 300TDT S123
1985 500SEC C126
2022 Sprinter 3500

And PMB does have the square seals if you do split the calipers. You'll find on their site.

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Owner of Mobil-Werks
Classic Sports and Specialty Automobile Service
1968 230 W110 +3.5 M116 & 4-Speed Manual
1985 300TDT S123
1985 500SEC C126
2022 Sprinter 3500

Using something like 500 grit wet/dry abrasive paper will casuse no issues. There is a lot of clearance between the bore and piston, so you won't be altering that with a bit of sanding.
The exterior seal lip can be cleaned with a toothbrush sized wire brush.

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84 300D, 82 Volvo 244Gl Diesel

Centric sells caliper rebuilds kits with the small O-ring square seal. I have used Centric caliper rebuild kits on the Mercedes and my 2 chevies.

https://centricparts.com/online-catalog

The issue with splitting the calipers is the torque. Mine have 12.9 blots on them. Many torque charts don't show the torque for that grade.

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84 300D, 82 Volvo 244Gl Diesel

Just for fun and to make sure we are talking the same thing. Your emory vs the emery I grew up with As it turns out, both are good!

https://wikidiff.com/emery/emory

I would be happy with a hone if I could get it in there. They even make 38mm brush hones. Problem with emery paper, is that small pieces of the carborundum come away and would need to totally removed. Same if media blasting was used.

Kent at ottosauce says to use 3M Scotch Brite. I have made a first try with that, but that yellow material seems solid. I have a few other options with stuff I have.
- Wrap Scotchbrite around a socket for tight fit and rotate by hand.
- See if I can use rotary 3M flap or brushes that I have with flex drive on rotary tool.
Mainly get cylinder surface clean and free of any "bumps" and especially, clean o-ring slot and the external rust where the boot fits.

and also read Pedro's link

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

I used the term wet/dry abrasive paper as I am not sure what the grit is made of. But, in general it does not shed as much of the grit off in use.

I am about 80 percent sure that emery paper is what is used in wood working and is typically beige to brown colored. Has to be used dry and while you can sand metal with it the grit comes off pretty quick compared to wet/dry.

With fine enough grit you could use either on the caliper bores, but the emery is going to take more and more carful cleaning.

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84 300D, 82 Volvo 244Gl Diesel

Subscribed to see how the yellow gunk is removed .

I've used Scotch Brite etc. taped to a drill bit, it makes a dandy hone that's very gentle .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

The last two times I did this I used a dremel with a small wire wheel on it to clean that area between the piston seal and the dust cap that gets crusty and causes sticking.
The wire wheel did not survive long, and it was kind of a pain to get the pieces all out of the bore, but it got the rust and gunk off quickly. There is a guy near me who breaks the caliper every time and reuses the square orings. He says he has never had an issue with that, but I don't have the nerve to try it.
It seems the 115 caliper kits often have the sqare orings, but the 123 kits rarely do.

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1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660