I recently bought a 3 year old A4 TDI with full Audi service history. Done 55k and registered Oct 2003! Despite seeing the dealers twice since new it had never had a break fluid service!!!

I had it done straight away and I must say noticed a very slight sharpening up of the brakes after.

I know it's due every 2 years. I guess the reason for this post is to say make sure you check it carefully when buying a used car and get it done if you cant see evidence of it.