If it were me I'd do the following:
1. Replace the calipers if they look original
2. Replace the brake hoses too
3. Since they are cheap, replace the master cylinder
4. Flush the system with new fluid and bleed, bleed, bleed
The only other component that could go wrong at that point is the prop valve, which is unlikely considering your issue is isolated to one wheel
Having good brakes is the most important part of your truck, so for under $100 you can have known-good brakes, at least up front (drums are another story) that you can rely on.
Before you spend any real money, you can try flushing out the old brake fluid and see if that improves anything, but be prepared to spend some money. I'm guessing you either have a collapsed hose or a bad M/C but see what happens with a good flush.