Try lengthening the top link all the way so the box blade rides on the rear blade only.
Bruce, this is standard practice and I did this many years ago when I was trying to make the box scraper work, but it made no difference--the box still filled up with gravel. Yesterday I removed the scarifier shanks, the front-facing blade, and the hinged rear gate, slightly over 150 lbs. total, tilted it all the way back with the top link fully extended, lowered it to the ground in float mode with only the rear-facing blade in use, and it still dug in deeply and collected massive amounts of gravel in the box.
As described by Land Pride in their user's manuals, the box scraper is made to move lots of material from place to place. If I could control the height of the box scraper with the 3-point hitch controls it might work. But every part of my driveway and especially the part that needs the most maintenance is either a convex or concave curve of varying radius, and the only thing that will work is allowing the implement, whatever it is, to float on the gravel surface.
Coming to terms with what I need to do to make this less work as I grow older, I am seriously thinking now of a grader scraper. It is made to float on its skids and would have to be used that way. But from several threads in TractorByNet I gather that these are also not without their problems, especially with loose gravel, so I need to research further.
If I need to go much farther with this I may start another thread. TractorByNet has several on the subject, but they are getting old and don't answer all of the concerns I have, and I find this forum to be very helpful.
A photo of the most difficult part of the drive, the ~350 ft. over a drainage area, is attached. The drop to the drain pipe is about 12 feet, so washout was a problem with heavy rains and a crusher run surface until I had it re-graveled, and the heavier gravel stopped the erosion completely, but it moves about more freely.
Thanks for your help, Dave Dalton