Could you explain that please? What have you identified in the ABS system which causes poor performance?

Actually weak point is external ESP pump for ABS 5.3 unit. Which make some internal bleed and lowering pressure to brake caliper. Which can be feel as sponge brake pedal. Now have ABS 5.7 from C5 latest year(2002-2005) have internal control for ESP and havent this kind of problems. Pedal is firm and sensitive. Hope to understand my poor english.

The biggest problem most of us have with the D2 brakes is that soft feel, which seems to be mostly down to the insecure mounting of the servo. Many of us have adopted MJs brake support block. This is exactly how mine is now after fitting it. A purely mechanical solution, quick and easy to fit. You might want to try it as well? +++

Start here for the full story: http://forum.a8parts.co.uk/showthread.php?t=1614
Continued here for UK people: http://forum.a8parts.co.uk/showthread.php?t=15947

That is interesting thank you +++ My 03 Allroad has 5.3 with the external pump and has awful brakes compared to my 03 A6 which has 5.something without the pump and has excellent brakes. It never occurred to me that the external pump could cause that. Tbh I suspected the single vs dual servo.

So, next question: what interaction does the cluster have with the ABS that requires changing to the C5 one? I thought it only talked to the ECU/TCU, other than warning lights?

Your English is fine, no worries :) I appreciate you indulging my curiosity.

I just looked at a VCDS scan of my (built in 2002) S8 and the ABS module (4D0 907 389 E) says version D57 which I'm guessing is 5.7 to you.
I wonder what versions were used over the years on S8s of various iterations?

Our sponsors have one for sale from a 2000 A8 3.7 which is a D46. https://www.prestigemotorservices.co...tem-16098.html

5.0 until 1997, 5.3 from then on, according to the documentation, although there are several variations within those models.

Could 5.7 have been introduced with FL.2?
Anything different under the bonnet that would be obvious to a novice like me?

FL1 and FL2 are all the same with the external return pump which, from Elsawin, is 5.3. The Allroad and C5 S6 prefacelift also use 5.3.

The 5.7 Nakata refers to turns up in the C5 A6 Facelift (02-05). This has ABS and ESP integrated in to the main pump block, with no external ECU or return pump. It makes sense that it would have a better pedal feel since it doesn't have the external pump or extra plumbing.

I'm entirely happy with decent calipers and a brake block, but that doesn't make it any less interesting or valid from an engineering perspective :)

So is that a red herring or misunderstanding by me? :o

Unrelated, just a firmware version I suspect.