Whether it's the same on the a4 or not I'm not sure bud.
I bought an A6 for the Mrs and the brakes were rock solid, I never felt confident with the brakes. I put a new servo in. Easy enough task. I could run you through it, but I'd have to have a look under the bonnet again just to refresh my memory. It took me about 1-2 hours.
It didn't fix the problem at all. URGH!!
What the problem turned out to be was...
On the rear of the engine, there is a fuel/vacuum pump. The outlet connection for the vacuum becomes loose over years. It's a pressed fitting over a metal connection. I took the vacuum unit off, pushed a good oil resistant sealant into the gap and carefully knocked over the edges of the unit with a light hammer. Prior to this, it moved around ALOT in its housing. Now it's rock solid and the brakes are fantastic.