I had my mechanic check my pads (which he told me they were fine) and I just recently had my master cylinder replaced because my brake pad warning light was periodically coming on. Now it is on all of the time without fail and it is making me nervous driving my car and I am constantly looking at that stupid light. There is no physical sign of pad failure (i.e. squealing or loud scraping noises). What could be causing it to stay on? Can sensors fail? Should I have my brakes checked out again? Thank you for any help you can give me.