1993 Mazda MPV brake system question

1993 Mazda MPV mileage: 87,000. When stepping on the brakes for an extended period of time (e.G, when stopped at a traffic light), the brakes go down lower, and the anti-lock light and brake lights come on. This happens intermittently. I would then tap on the brakes again to make the lights go away. At least three different mechanics (including the dealer) checked out the brake system and told me that there were no leaks and the brakes were in good working order. The dealer also told me that it was either the anti-lock controller unit or the master cylinder, but that they couldn't tell for sure until the problem got worse. One mechanic checked it out with a computer and found nothing wrong with the anti-lock system. Shall I just wait until it gets worse or go ahead and replace both the master cylinder and the ABS controller unit?