Brake Booster Adjustment?

[GregATL]

Sorry for the ignorance but I have a question that I am hoping someone knows the answer to. On our 99 Boxster the brakes got very hard to push all of a sudden one day. Since the car is still covered under warranty we took it in to have it repaired. My feelings are that the brake booster failed. The story we were given was that the brakes needed an adjustment before they could determine if the booster was bad. They said the booster would be covered under warranty but the adjustment would not. Price for the adjustment $600.00.
Now I have done a number of complete brake overhauls on my BMW's over the years including the hydraulic assisted system on my 535 and this just doesn't sound right to me. Does anyone know what this adjustment may be and what would be involved in doing it. Additionally, it seems to me that if it were an adjustment it would have been a progressive problem not one that happened suddenly. Any help or opinions on this matter would be appreciated. Thanks.