Citroen C5 dashboard warning messages

driving along, suddenly a ping sound and dashboard warning ANTIPOLLUTION FAULT springs up, and engine light and 'service' illuminated on the instrument part of dashboard..stop car turn of engine, re start and fault warning is gone ? it then does it again either next min next hour or next day
NOW IM getting another ping, a big red STOP and parking hand break light comes on !
stopped checked hand book, low break fluid level....upon check the resivor its not low at all. wait few mins,,,re start,,,the break failure message has gone..??
turn on radio and heater,,,,some times it comes on again somne times it dosent. go for a few days than starts again


any clues ???