1995 Chevy Cavalier ABS problem

Brakes problem
1995 Chevy Cavalier 4 cyl Front Wheel Drive Automatic

About a year ago my ABS light came on. Had a new speed sensor put on but that didn't solve the problem. At the time, I couldn't afford to have anymore done, so I've been driving since then with no ABS/light on, etc. About a month ago, immediately after a heavy rain, I noticed the light wouldn't come on, and as I would come to a stop the ABS would click rapidly, sometimes very rapidly when making a sharp turn into a driveway, etc. Also, I'd notice a very marked drop in brake pedal pressure at times as well. If I'd let up and reapply the brakes the pressure would return enough to stop the car. A friend at work who is pretty knowledgeable about cars said this could the ABS control module? Also, am I doing more damage to the ABS system by driving it in this condition?

Thanks, :)
Jeff