Sharing...........

Original post is here : https://mbworld.org/forums/e-class-w...iston-ate.html

I found out something interesting on how the piston seal is set to sit on its cavity.
I did not pick this up when I replaced the front 4 pistons Brembo piston seal and dust boot, but I can see better with the ATE single piston rear caliper.

The 4 piston brembo piston seal cavity, due to angle of my photo, it is not 100% clear of the mini ridge at the cavity.



Single piston ATE rear caliper, it is much easier to see the piston seal cavity is not 100% equal like typical o-ring cavity, it has mini ridge.



Illustration of mini ridge below, I hope it is clear.




That is why the seal has sign of different compressive force on it, from the mini ridge. See the line below.


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I have been wondering on how ACTUALLY a piston seal is supposed to do its auto rectraction.... to lift up brake pad from touching rotor disc, when we release brake pedal push.




See minute 04:22 upward

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The piston itself is super smooth outer wall, so for seal to grip it a 100% is not an issue.
Now I realized the mini ridge at the seal cavity caliper body side is there to ease the piston seal pulling back the piston inward.

The lower part or the bigger diameter of the seal cavity region is some sort less grippy cavity and brake fluid will enter or can be released from this tiny zone. Its like a small angle "hinge".
The higher part or the smaller diameter of the seal cavity has most grip. I hope I am making sense





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Ate original dust boot, since 2014


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The bottom part is actually a steel ring, cast into the rubber. This is how the dust boot can stay well on the caliper.



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If we simply test the piston to seal tightness like below, one may think why so loose the seal is to the piston body O.D ?
It will slide up or down by simply flipping the piston around.



I have lots of experience with o-ring on piston and air driven, but not square seal hydraulic driven piston.
Also the o-ring is the dynamic one, the moving one along with the piston.
This brake system is the square seal is static, the piston is the one moving.

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Anyway, it is a perfect fit. this Brembo kit.

In Brembo wesbite, ATE system is called TEVES system.
ATE = Alfred Teves , the same.


I managed to do Left rear today.



Very nice inside, no corrossion or nick at all.




The new piston square seal is a tiny bit softer than the old one inside the caliper. But the old square seal is by no mean hard or brittle.
The old dust boot is also not hard or brittle....amazing for 11 years old, albeit only 44,000KM.

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I never done any caliper teardown other than my 2 front 4 pistons Brembo-s in Sept 2022 and now the rear ATE one.
Never I owned a car this old, 11 years old today my E400 and the only car I DIY the most.

Other cars of mine, I go to my cousin garage or official dealer, I dont bother to DIY. I rather DIY on yachts.

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So by this week ending, all 4 of my brake caliper pistons's seal and dust boot are not older than 3 years or 7 days.
All refresh-ded. Feel safer.....

 

As to why is the groove have a stepped backing - no idea other then it allows the square ring to squeeze in more.
Maybe this reduces the drag on the piston or promotes better piston retraction or something that Brembo thinks makes theirs better, lower costs and/or reduces warranty issues?

I would not lose sleep over it as many ways to SEAL the CAT so to speak -

AS an FYI the Sealing Guidebook by PARKER HANNIFIN INC - the goto seal design book:
ORD-5700.pdf

Discusses material, static and dynamic sealing.
the Appendix is great as discusses Failure modes, Assy etc.

 

Thank you Sir
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I know that book, I always thought it was purely for o-ring..
Let me take a second look.
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Just as thought all will be well.......... I found a new challenge.

New rotor disc and new brake pads, meant my caliper piston now sits very deep, to allow maximum "jaw" opening.

For some reason unkown, my piston retracting behaviour is poor when piston got so deep, but good when piston is not too deep.
I called my indie and ask him, how often did he come across this kind of weird piston behaviour ?
He told me he already found a few MB with this behaviour for the single piston type caliper.
He thinned down the brake pads, no choice he said.

My removed old rotor is 21mm , down from 22mm. 1 mm free space.
My removed old brake brake pads are now 13mm max both both, while new one is 16.5mm ish. 7mm free space here.
So the piston seating depth with oldie parts was not too deep....due to the 8mm gap allowance or piston insertion is less deeper by 8mm.

Here is the piston seating depth using 20mm wooden block and not the 22mm new disc rotor and approx 6mm nylon sheet.
With 20mm wooden block the piston retraction is decently nice. With approx 6mm nylon sheet, the piston retraction is very good.
With 22mm rotor, the piston retraction is poor and I get 30Nm drag.




Super smooth healthy piston and caliper housing and new piston seal.... what's actually going on ?
I am beginning to hate single piston caliper even more. I already dislike it when I first tried zero touch brake pad adjustment on it.

This is the first time the rear caliper actually has to do maximum jaw opening, other than when it was first assembled in 2014.
My indie replaced my rear brake pad in 2019 at 20,100KM and the rotor disc was not replaced, but I know it has gone down at least 0.75mm to 21.25mm at least.

Below is the thinning down of the brake pad at 10,000KM and at 20,000 KM. This is the original brake pad to the car since 2014 and not the 1st replacement in 2019 November.



The November 2019 new pad, is now this thin : 13mm




Brembo or MB new one is typical 16.5mm


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Stupid dumb-azz single piston caliper, the brake pad true wear is not equal between the two partnering pads.
The main pad is thinner, the one the piston pushes, it has the steel claw thingy.
The follower pad is average 0.5 to 0.7 mm thicker. You can tell from the above 2 photos.


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So I tested the LEFT side with the oldie brake pad.
Both are approx 13mm only, not 16.5mm thick like new Brembo.

The piston retraction is beautiful. No drag of 30Nm anymore.

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Looks like I have no choice but to sand down the Brembo brake pad to say 15mm each. Or loose 1.5mm each.
, its not the loss of 1.5mm, its the amount of work I hate the most.

I did try to use the rotor disc as "sandpaper" using engine and transmission and braking power on Dyno mode, but I do not know how long 1.5mm will need.
I tried like 20 ish minutes and up to 146C rotor disc temperature. I do not want to exceed 200C. There is no weight of the car as momentum.
I also do not want to do it on the road, as I can't control the rotor disc temperature.




Left is the worse offender.





2 more CM up for the wheel bearing hub, it will be actual ride height. This way the cv drive shaft will last longer by working in a more straight line while loaded by the brake.


At ride height, the spring arm is near straight.




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One more thing.
The suspension will "twitch" when brake is applied hard to a stop.
If one has very corroded subframe and suspension arms are no more properly supported, I won't be surprised if upon braking.......... the car can be very wild and dangerous.

 

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Further test I posted here : https://mbworld.org/forums/e-class-w...ave-agree.html

Conclusion :

Proven that the piston insertion depth matters. This meant my piston is no more good at all insertion depth, more so at maximum insertion depth.

I am getting my cousin garage, his boys to sand down the brake pad to 15mm from the 16.5mm new, by hand slowly on a flat surface, like a glass.
Me too lazy to do it myself and the close by machine shop don't like weird small job like this.
I tried some years ago using the close by machine shop to remove front brake pad damage region due to overheat at the track, the result was flat but angled

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I think next best move for me is to buy a new caliper piston.
Brembo is selling the Budweg brand for this ATE piston, so it seems.
https://www.bremboparts.com/europe/e...ngkit/F_PI_066





I just ordered 2 pcs, from the same company I bought the seal kit from in Latvia Europe, hope it will arrive 1st week December 2025.

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I think the seal compression region at the top region of the piston has been dry, I mean not brake fluid bath for like 4 years now from pad no more new, its hidden under the dust boot only.
Albeit looking smooth, actual surface finish for the seal to grip well for retraction, we can't see. Sure, no fluid leak.


Some sort of very mild discoloration can be seen though.

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With the new pistons I can buy new brembo brake pad set again and re-try again.
I don't like this un-finished business.
I mean the caliper must mechanically behave like a new unit, upon succesful rebuilt.

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Brake pads thinned down to 15.x mm from 16.5mm new.
Amazing what +-1.5mm can do for the piston. The retraction is beautiful now.

The sanding result for brake pads are decent, not the greatest.
At the least now torque value for hand spinning both L or R sides wheel hub is only 1-2 Nm apart.
Temperature value right has better sanding result and is 11C hotter at 170C vs left at 159C , rotor vent temperature.

I tried bedding the pads using dyno mode. Not bad. But take lots of time.
Cooling down from 170C to 58C need more than 30 minutes. Max under 200C is my target.



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Marker pen does not lie, left side .......outer brake pad touch zone is not as good as right side. Hence 11C cooler temperature.
I did not mark the inner side of the rotor.



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Since driving the car will yield supposedly only 30% brake force at the rear, and brake test on the road is a danger to others,
I guess using dyno mode is a better choice.

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Left and Right caliper pistons retraction + 2 bedding process from rotor and instrument cluster point of view.


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Will need to do bedding to 170ish Celcius at least 3 more sessions using dyno mode, to give brake pad surface a more proper equal wear.
With cooling down period yada yada, this will take at least 45-60 minutes per session.
What I do not like is the engine is like being driven short distances, not too good for the engine.
But If I do this kind of brake test on the road, people will think I am drunk and I can't easily monitor rotor temperature.
If rear gets to 170C hot, I guess the front ones may be as high as 250C, as supposedly front does 70% of the overall braking load and the rear only get 30%.
So stuck using dyno mode ( off the ground ) as best controlled test.


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