For finish grading, am I dragging the box with the 3PT all the way down so the full weight of the implement bears down on the surface?
Yes, that's exactly what I do.
I've found that when I've dragged the box with its weight on the surface it tends to fill up. I suspect that this is because my top link is not adjusted right.
Yes, you are right. You might need to tilt the box backwards a bit to keep the front blade from cutting/digging out more material or dirt. You will want to have the box "ride" a little more on the rear blade so the rear blade will crush the material down and smooth it.
I didn't know you have a hinged rear, so consider fastening it while doing this step. That way it should support the weight of the box and smooth out the material. You can see if it's hinged, it could swing out of the way and allow the front blade to continue cutting or scraping.
Just to be clear, you are saying I should be able to find a top link position where I can have the full weight of the box on a gravel surface and drag it to finish grade without immediately filling up the box. Correct?
Exactly, but remember in your case that the rear blade needs to be fixed so it can smooth and not flop out of the way, allowing the front blade to dig down and cut again. Now remember I don't have a hinged rear box blade, so you will need to tell me if that works for you. I know it does for my fixed rear blade boxblade.
Anyway, the rear blade should support the weight of the box and smooth the material. It should keep the front from digging in or scraping too much. You want to set the two blades so the front barely cuts, if any at all. Just to knock down little ridges or pick up some stones, but not cut the entire surface. At that point you want to be smoothing with the rear blade primarily.
Once you get it right, a light will come on in your head and you'll know exactly what I'm talking about and you'll say to yourself, "Self...uh so that's how you do it. That's so easy why didn't I think of that?"
You''ll see the difference right away.
Let us know when that happens. When your get that part down, there is no stopping you to making perfect roads, trails, pads etc.