2001 Chevy Silverado brake lights come on

The Brake lights on the dash (idiot lights)come on after a few moments of driving but when i stop and turn off truck and start it back up again the lights are off the rest of the day. some days the lights on dash dont come on at all and some days two or three times a day . my ? is when both lights come. what is the problem. i cant find anyone with the answer. the book says there is a problem with abs or there is a problem with parking brakes but not both. why do both come on. I have changed all brakes and wireing and sensors still the lights come on. its been a month now .