brake issues - replaced calipers, booster, ABS pressure sensor 1 fault code

[Performify]

Have a fun problem the tech team says they've never seen before.

As part of a comprehensive refresh and engine out we replaced all four calipers with rebuilt ones, and also replaced the brake booster with a NOS.

Brakes were afaik working just fine with no codes before we started all this work.

Here's the writeup. Working on getting more detail. Have also suggested from research here checking the magnetic hub sensors (https://www.rs6.com/archive/index.php/t-21095.html) and replacing the BLS (https://www.flickr.com/photos/surgenor/sets/72157623008684652/detail/) (part here (https://www.audiusaparts.com/oem-parts/audi-brake-light-switch-1k2945511rdw)) per threads and the FAQ here.

tech's writeup:

Power Bled brakes once. Completely non functional.

Power bled again using more pressure and fluid, front left and rear right caliper function. No ABS.

Power bled and manual bled all calipers on reverse order as recommended by prodemand, still no ABS and only 2 calipers function.

Bench tested calipers to verify function. Manually bled the master cylinder and all ABS module lines, abs pressure sensor etc.
Abs module does not have electronic bleed procedure. Power bled calipers again, all brakes function well, car stops like normal.

Abs and esp still do not function. At this point the brakes stick on until pressure is relieved.

Released pressure and power bled once more. Brakes still function well, abs and esp still non functioning, brakes begin to stick on the more they are used.

Inconsistent abs fault code is 'ABS pressure sensor 1'.

Replacing sensor to clear fault. And probably the BLS and cleaning the hub sensors just to make sure.

If fault is not cleared with new sensor, ABS module is likely faulty?

Anyone with any experience with the rebuilding services, e.g. ABS.de (https://ecu.eu/audi/abs---asr---esp-control-unit/esp-1.34/audi--4b0614517ad-bosch-0265950083-0265225167-102050.html?__cf_chl_captcha_tk__=4e986c777a93f87f d96a3e7cf27f75e2860a7761-1591400742-0-ATv2Oto1AegZ-Xkn941s7yC4q-RvZhAIXUtFQ2UxVfxmf-FzBMdMv_bMjTzOF0RZug55jA60WyjH9S1L_0rC7r84HP-wcYJ7dziUmSTFdGlHjkNF2K-kXwtdhLcg3hTqwuQeztLDHnrm1RPlpUUGWqgkUW-5jkvQ6mE9sqTtnAV3TpAcWCGW1qM8WBT0MpeGvP7AcRRNsnLfF 91-4z9v94qmrhNzQJpMginhpfdCfz0L14bXr_709pGk1vwJBNxMD8 xcdOebuGKC9QZgeM4IR12srqI9j5ytA-rns_qv6XR7Udzshw2K-SiriB2DkYOTV1n4TI0w743O0JytNWtLI-TeMwf9xMpLLI4Od854MT4KmxMye7QNB8TRNVKSX_HUg77LEe2W ImyrjQumO-eYtAJXxmn4fOEK8wwiWJuP-aTe7KHIHkJPXcPOdlo22AXxKAKkCA521t5CTjQDKH-ToNO58IWvWRUWtwi2YPkKTUQR_AsIy-A7-5F_7GNAfV_z0x6-GOEq1zq7vMzuGyp5NJyAxqc8Q4U3L-AcYJEvknPCJc7Z3aju-2Xbhgso92BHGGp6yMh8ebzXLxDS7b5y73PqYiZ6IK4-PCvl0B6y9QapFwmPM9GX5ez2uf514S0ZvQ)? Cost certainly looks positive compared to buying one...

Anyone any other thoughts, ideas, etc? Will report back hopefully with additional troubleshooting detail, as well as results...

[nene]

Some recommendations:

1. The brake pad could just be the side rails on the caliper. Are any of the brakes sort of dragging? The grease on the sides on the brake caliper may gone 'gummy' (technical term!). You can try and get a refresh kit really cheap, rebuild it with correct grease for brake calipers and it could solve the problem.

2. Brake light switch that has gone bad. Try logging the Engine or Transmission for brake light switching.
Drive with the Cruise Control (CCS) on and see if the system works as per design. If the CCS will not operate or the CCS does not shut off when the brakes are applied the Brake Light Switch may be faulty.

[Performify]

thanks nene.

so far no luck.

pulled the ABS module and sent it to a rebuilder -- they sent it back with "opened it up, tested fine, no issues here"

Replaced the pressure sensor and brake light switch already based on the feedback above.

Brake booster was known bad and replaced with NOS very early on, so we're going to pull and check that next in case it was faulty.

[Performify]

Can anyone confirm the correct part number for the RS6 brake booster?

we called Audi and they're saying 4B3612107C.

ECS website lists 4B3612107A (says that the -C is for 2004+ and the -A is for 2003) however the ECS website also lists a VIN cutover of "From VIN 4B-4-903201" -- mine is 4B-13N-905341 so I'm honestly not sure (the 4B-4 vs. 4B-13 is what throws me).

Looks like we replaced it with a 4B3612107 (no letter) which is not the correct part for the RS6 and is what is behind the issues.
(from here https://www.europaparts.com/brake-booster-4b3612107.html)

edit: further conversations with Audi parts we went ahead and ordered the -C variant which shows as a replacement for the -A. It's $200 more but at this point don't want to spend any further money/time fighting issues and get the beast back on the road. Will update the thread if this does / does not solve the issue.