I wouldn't dismiss the load proportioning valve but in theory, a whack with a mallet shouldn't cause a pressure release caused by a component at the other end of the truck. I think you need to blow the piston out of the caliper and give the inner reservoir and walls etc a good cleaning. I would also consider replacing the rubber brake lines. They can come apart internally as well + years of service can weaken them to the point where they expand and drop pressure. I hate messing with brakes. They can be temperamental and they are kind of important to get right lol.