Mark & Bird: Thanks for the comments. That's why I ask questions. I'm clear on adjusting the box angle for forward and backward cutting/spreading operations. However, this is the second time I've been told that position control doesn't hold down a 3ph--actually I guess things probably would break if it was held down.
So, now the idea finally gets through my skull. My 3ph always can float up. It just seemed to be held down when I buried implements after starting up hills. The explanation probably is that a sudden change in grade has the effect of changing the cutter angle to the ground. What actually happens probably depends on whether the side plates dig in or track the new grade.
After taking yet another tour through oil flow diagrams in my repair manual, I found a missing, and very simple, idea that escaped me. A 3ph-lift cylinder only has oil on one side of it. So, no oil has to flow anywhere when a 3ph floats up, it can just slosh around in the cylinder like a half-full bucket. That's a DUH, huh? Anyway, whether or not the 3ph is held down, the box tilted down so it rides on the back of its rear cutter still does a good job of compacting gravel.
These comments have straightened out the idea for me, and I hope cleared up the subject for others. There are more complicated issues like the effect of unloading and unloading several valves from repeatedly floating a 3ph. Whether some oil is lost, which changes the position, or whether any lost oil is automatically restored? Guess I can go back to my infernal oil flow diagrams to study on this one.
I guess the advantage of draft control is that it gives a power lift, rather than a float lift, which is adjustable rather than entirely dependant on the implement weight. I guess draft control 3ph’s can float as well as power up.
