2003 Nissan Murano brakes problems

2003 murano was stopped. Driver had foot on brake. Vehicle was struck in rear. Brake pedal went to floor and vehicle rolled through intersection. Had to pump brake pedal twice before brakes would hold again. Ever since, vehicle labors as if pulling a heavy load or as if parking brake was partially engaged. Nissan dealer states that brakes seem to work, therefore they will not attempt to find cause of pedal-to-the floor incident or cause of vehicle "laboring".

As I am driving, our vehicle will auto brake, bow up and lose acceleration causing traffic following behind me to either brake or get into the other lane to keep from hitting me. This happens at low levels of speed and at high levels of speed. This will also happen should I be trying to cross traffic as well. I am in fear for the safety of my family and kids as well as the safety of other drivers on the road for they may not be able to get stopped in time.