Why are my brakes locking up when it is raining?

I have a 86 gmc sierra 3/4 ton. The brakes completely lock up whenever I touch the brake pedal even slightly. This only happens when it is raining or just really wet. And also when it is dry the truck will not stop, I have to push the brake pedal really hard before I even start to slow down, and it takes forever to come to a complete stop. I dont know if I need new calipers or just new brake pads. Any feed back would be greatly appreciated.