Toyota Repair: Prius Brake Booster, brake booster, toyota dealer

Question
We have an 01 with 110K. I had the brake booster replaced (about 2K!!) as the codes retrieved (C1252, C1256) indicated a malfunctioning booster and acumulator. Since the repair the car will start to creep forward shortly after coming to a stop with the brakes applied normally. We have taken the car back to the dealer but they could not find anything wrong and rebled the brake system. A 2nd visit to the dealer and now they say it is the master cylinder. For the 1st repair I stipulated that the $2K repair must fix the problem. Now they want $1600 to repair the master cylinder. Unfortunately we don't have many options here on the Big Island of Hawaii and the mounting cost is going beyond our means. We have lived with it for a few months now. Can an independant replace the part? Should the dealer honor the repair?

Answer
If the same trouble codes are not present again it seems that the dealer diagnosed that part correctly by replacing the brake booster but it also seems that there is another problem, are there any other trouble codes now, are there any warning lights on the dash on? This is an extremely complicated system and I would not recommend having anyone but the toyota dealer work on it, you may want to try to get another opinion from another dealer if that's possible.