2002 Mercury Sable, wierd brake problem
 

My wife was driving the other day and she said when on the downside of an overpass she applied the brake and it went mush and some dash light indicators came on after a few seconds the brake pedal was firm again and the lights went off. It has been okay ever since. I checked the master cylinder fluid and also the battery connections and a few other minor things, nothing seems amise. Any ideas on what could be the problem?
Many thanks