Brakes on a 1984 P30 postal truck.

Hi, I have a 1984 Chevrolet P30 Postal Truck with 144,000 miles on a 6.2 liter diesel engine. Since there is rarely any place to ask questions on a truck like mine, I'll use the truck repair section... The front driver side brake isn't engaging when I press the brake peddle. I was driving the truck, and it started shimmying like the front end was coming out from under the truck. After $275.00 worth of a tow bill, I took off the wheel, and sure enough, the brake will not stop the wheel if I'm pressing the peddle to the floor. First off, how do I get the caliper off, and secondly, what would make this happen? Got any kind of advise for me to make this brake work??