Air Brake Abs Warning Light problems of the 2001 Mazda Tribute

My brake light keeps coming on and going off while driving. No apparent braking problems. Dealer says I need new brakes and master cylinder without even inspecting. When I mentioned the many complaints regarding brakes on this web site for this make/model vehicle, he was not concerned - just said he was sure that is what the problem is. I have less than 20,000 miles on my car - have made several long trips but very little stop and go driving. I find it impossible to believe I should need new brakes and master cylinder. Will have assessed by independent service provider.