Problem with Brakes

I recently purchased a 1995 ford explorer (137,000 miles). I am having a problem with the ABS system. Just before I come to a complete stop I feel the brakes pushing back and making a hissing sound. I'm not having any issues stopping at all. This morning the brakes didn't push back but the ABS light came on. Now the ABS light is off again and the brake is pushing back again. I've been told it's many issues from a bad sensor to air in the brakes.