Volvo Repair: DSTC/ABS problems, faulty code, brake failure

Question
I have a 2000 S-80 For as long as I have owned the car the DSTC light periodically comes on.  Several trips to the dealer yielded no help because of no lock-in codes(car was still under warranty).  My recent trip to the dealer showed a locked in code (and the ABS/DSTC service needed light was on) they said the pressure sensor brake booster needed replacing($770.00 and the car is past its warranty). The DSTC light still periodically comes on however the ABS and DSTC functions still work(I tested them on the road)My question is can the computer show a problem code without an actual problem with the ABS/DSTC and what could cause this?

Answer
Hello,
firstly we should understand that "periodically comes on DTSC light" is a faulty or "normal" job?
I mean, maybe as you know DSTC lamp lights in certain conditions allready, for example if the tyres slip, or if the car has looses its stability because of very "hard" highway curve, like this...This lamp should light in these type conditions, it means the system (DSTC system) works. In these times, the DTSC lamp "flashes" and then goes off.
If DSTC system and so ABS system has problem, on the Driver information module left side there should be a message as "DSTC system service needed" or "Brake failure Service Urgent" like this... Also, if the system has faulty, certainly and certainly there is faulty code in ABS Control Module, and the dealer can find it, which code is stored in ABS Module and which part is defective.
Firstly, according to writings given above decide which type happens on your car, "problem" or "normal job" (I am speaking about periodical DSTC lamp)
Secondly, as I can undertand from your writings, there is a faulty code about a sensor, "brake booster pressure sensor", but I could not understand which it is?
Because you gave price ($770), I think it is price of brake booster. And, on the brake booster there is a sensor called "brake pedal travel sensor" and approx price is $100 or less. Also, I should write that (ý think this will be an answer also for your last question) when the lamp ABS/DSTC lights, the system does not work, but whenever it goes off, the system starts to work again, I mean because of some parts the system shows faulty but if this faulty is intermittent when it is "off" the system will be "on" again.
I hope this info will be beneficial for you.
Also, if you give me some info about DTC (Diagnostic trouble code) the dealer found, I can help you more, I mean if you give me the code I can tell you what you should do for future...
Best Regards//Vedat...