brake lite on 1991 Chevrolet G Series Van (G20)

When I backed out of my drive way this morning my brake lite came on. What is the most common reason? Dealer just checked my brakes for wear and said front brakes pads were 70% worn and the back shoes were 65% worn. Brake fluid is full.

Asked by for the 1991 Chevrolet G Series Van (G20)
Air in the lines, bad master cylinder, bad wheel cylinder, bad brake line, etc. Dangerous situation, have vehicle towed to a shop for diagnosis and repair.