Passat-brake fluid and BAS warning light on

Hi, I drive a Passat -98 (variant v6 syncro) which started to set the brake fluid and ABS warnings lights on. It started first with brake fluid and after couple of week the orange ABS light was simulaneously turned on also. Sometimes I can drive couple of km's before the lights come on and sometimes they are activated directly when starting. They are also sometimes turned off when driving just to be turned on again after a while. The level of brake fluid is naturally checked as well the sensor which works ok. I suppose the problem seems to be in those 'black boxes' then? Have anybody suffered of similary problems? Do anybody know what's the price range of a brake/abs black box? -peppe