I'd have a good look under the truck. Trace the brake lines to make sure nothing broken and leaking if it's been sitting for a long time (especially on grass or farm field). My '88 was sitting out in a field for 3 years. Underneath, it looks like it was pulled off the Titanic. Every line rusted through (or close to). The flex lines were probably ok but as the hard lines were broken, the lines were fused into the flex lines.
Check the rear wheel cylinders too for leaks. If one drum is really dirty looking inside, chances are it's a leaking wheel cylinder. I agree with the others on the front brake. Probably flexline is bad if you can still move the caliper.
I have to say, the brakes on these trucks are overly complicated to do. Lines are easy to get and parts are cheap if you're thinking of a do-over.