Dodge Grand Caravan Brakes Click and Hard to Push

Our 2005 Dodge Grand Caravan SXT (42000 miles, 3.8 L engine) needed O2 sensors replaced. My husband did this as he is pretty proficient with auto maintenance and repair. The problem: After he replaced the upstream O2 sensor and changed the oil (routine maintenance), he drove to town (about a 20-30 minute round trip).

On the way home, the brake pedal became almost completely unresponsive. He had to push down with all his strength to get the van to stop. There is a definite clicking noise whenever the brake pedal is depressed. He went back and checked everything that he could think of, including the brake fluid (fine). There were absolutely NO problems before he changed the oil and O2 sensor.

What could be causing this all of a sudden? Your immediate response is GREATLY appreciated!