You need vagcom to properly diagnose the issue. It can be as simple as a faulty brake light switch to faulty ABS unit. All depends...
2004 V70R M66 - Current
2001 A6 Avant - Past
2.8 ATQ, Quattro, Tiptronic
Mods - Was chipped, but not now... Was using K&N, but not now...
Repairs - Too many to list! And you wonder why I still drive an Audi
So I don't drive one now.