I have an 2000 Silverado LS and every once in a while the ABS and Brake light come on at startup. The manual says to have it checked by a mechanic. I have replaced all the rotors and pads. Can anyone give me an idea of it might be?

 

Mine did that for years before I sold it. You might try cleaning or replacing the speed sensors. Sometimes it works, sometimes it doesn't. At worst you will have to replace the ABS unit. The brakes still work. You just don't have ABS when the lights are on.