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At first I thought the ATE rear caliper piston is aluminum like front Brembo 4 pistons caliper.
But the more I think how crazy light it was and its fine details, and some google search ( the AI maybe wrong though ) stated : phenolic piston
See the rear end, I circled green with question mark.
FRONT Caliper, aluminum guaranteed. Its different.
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The Brembo supplied rear caliper piston for ATE and by BUDWEG, just arrived. F PI 066 Piston Caliper
It is steel, and coated with chrome I guess.
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I was suprised by the heavy 407 grams of weight of the Budweg....hence it triggered the question.... on if the original ATE piston is phenolic type because it was crazy light ? I never seen phenolic type caliper piston.
Aluminum is 2.7 grams per cm3, steel is 7.85 grams per cm3, whle phenolic caliper piston is only 1.35 to 2.13 grams per cm3.
This is what I do not like about composite material, it will absorb water and that meant it can swell if we submerged it long time enough in the said liquid.
When I replace the phenolic caliper piston with the steel Budweg and if all goes good for say 1,000KM, I will drill the old ATE caliper piston.
I can tell from the drill bit dirt if it is not aluminum.
New Brembo rear brake pad on order ( again, 2nd time ), because I already sanded down the first purchase one by 1.5mm off to make the oldie piston able to retract properly.
So I can start the experiment if possible this week with 100% new thick Brembo pad and while my rotor disc is still new-virgin thick and solve the mystery once and for all.
Damn-it, indeed the ATE piston it is phenolic, PLASTIC !!!!
You guys with single piston or dual piston but floating front caliper.......... may check, if you also get phenolic piston.
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407 grams the Budweg steel piston vs 106 grams ATE phenolic.
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Yep, the phenolic piston swell at least by 0.04mm at the top side. This is why when 100% thick new brake bad and new rotor, piston did not retract properly.
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Budweg steel piston installed.
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New Brembo, vs the one I sanded down by 1.xx mm
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Knock-Off / Fake MB brake pad wear sensor , the new one. Not bad, it works ( too simple a product to NOT work ) and decent quality. US$3.00 vs genuine at US$19.00 .....
I insert-remove-insert the sensor so many times during this rear brake pad ordeal, it simply gave up ....LOL.
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The happy ending.......... Good piston retraction.
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I did not even bother to bed down the new Brembo rear brake pad. Too lazy today.
The newton resistance , which is torque to initiate spin of my rotor disc and my LSD DIFF.... meaning the other wheel spin too....the reading is low enough.
This reading also included the parking brake pad minor touch if any.
Right braking system 25.5 Nm , tranny NEUTRAL, parking brake zero click. Clockwise spin for Right side to simulate car moving forward.
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LEFT braking system 22/7 Nm , tranny NEUTRAL, parking brake zero click. Counter-clockwise spin for Left to simulate car moving forward.
The phenolic piston with new Brembo 100% thick brake pad not sanded and new virgin rotor was giving 45 to 50Nm of drag, due to the phenolic piston did not retract properly.
11th Nov 2025 archive............
Phenolic piston and 100% new thickness Brembo brake pad not sanded down. RIGHT SIDE. 45Nm
11th Nov 2025 archive............
Phenolic piston and 100% new thickness Brembo brake pad not sanded down. LEFT SIDE. 53Nm
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So case closed.
Phenolic piston sucks !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! if already at 10 years or older.
Wasted my time and my money...............dugggh.
But, the good thing is, I learn something new....... .
Last edited by S-Prihadi; Nov 28, 2025 at 10:15 AM.
Ford uses these excellent plastic brake pistons that swell with heat... cheap replacement for rusty-steel pistons instead of nickel plating.
My front basic floating caliper uses a single steel piston.
> STOCK SETUP...
The only thing bad is a strong-spring that disables proper caliper centering regardless of silicon lubbed slidders. This reduces brake effectiveness by nearly 50% front + back... One pad hardly works at all!
Last edited by CaliBenzDriver; Nov 28, 2025 at 03:23 PM.
Single piston caliper = SUCKS !!!
That is the only comment I have for them.
- Crude, never balanced on the pressure at both sides of the pad. Pad with piston will wear out first.
- Piston retraction is so minute and is expected to pull back the entire freakin' sliding assy ? Hello ....cheapo cheapo sh-iet.
- Can never produce zero touch like the front Brembo 4 pistons.
Really, manufacturer can add another US$400 to the car price tag and get the Brembo 4 pistons or whatever other respectable brand as long as not floating caliper with pistons/s on one side only.
Last edited by S-Prihadi; Nov 29, 2025 at 05:27 AM.
Reason: add info
Mercedes SLR McLaren 722 S Is Extremely Rare Example Modified by McLaren
Slideshow: A one-of-one U.S.-spec Mercedes-Benz SLR McLaren Roadster became even rarer after a factory-backed transformation at McLaren's headquarters.
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