Thanks guys for all the help. It took a while, mostly waiting for help bleeding the brakes, but they work just fine now.
I ended up pulling the wheel cylinder off and cleaning it. The internal spring had a small amount of red rust and the cylinder and pistons were lightly stained in a few spots (no scores or pits). I went over them with 600 grit wet sandpaper. I also found that the top edge of both pistons had a very small burr that I carefully filed smooth. I checked all of the linkage pivot points (they were all ok). I cleaned all of the components with mineral spirits and readjusted the brakes. So…………I’m really not sure what was causing the locking problem. Which probably means it will come back again, but then I wouldn’t have an old truck if I did not like working with it.
