A couple pieces of information would be useful to sort out the problem. First, will the loader definitely not raise when it won't lower? Second, can the relief valve be heard when trying to lower the bucket? Does the 3ph lift when the loader doesn't work? And, it's not clear if the bucket lowers immediately, or after, several minutes after, shutdown. These things should be determined when the power steering isn't on. I guess another question is whether the power steering can be heard when the bucket won't lower.
Couple of possibilities: The easy one is that it's a priority problem. The 3ph may be lifting (even slightly), so the loader doesn't work. Whether the 3ph get priority depends on the tractor. However, with that sort of thing, the loader probably would lift either, and the relief valve wouldn't open.
I had nearly the thing you describe once when I didn't get a quick connect fitting firmly seated. The relief valve could be heard immediately when the lever was pulled one way, and after a short delay when pulled the other way. You might check any quick connectors. Note that a tractor shouldn't be run very long when the relief valve is opening--it's hard on the pump and the oil heats up.
A third easy thing to check is whether the power steering is working when the loader doesn't. There is a priority valve that takes oil for the steering first. In a normally working hydraulic system, the steering shouldn't take enough oil to prevent other hydraulics from working, but is a long shot possibility.
A bucket should lower when the engine is off. It doesn't need pressure, the weight should be enough to cause return oil flow from the cylinder through the return lines to the sump. If not, there is an obstruction. Note that the bucket lower return lines are also the bucket lift pressure lines. Many types of obstructions that would prevent a bucket lowering would also interfere with the lift.
I believe the float position simply connects the pressure and return lines in the circuit so oil can flow from one side of the cylinder to the other as the bucket follows the contour of the ground. The fact that the float position lowers the bucket when the bucket circuit won't probably eliminates the cylinders, lines and connectors as sources of the problem. The final easy thing is to change the oil and filter if it hasn't been done recently. I got a piece of rubber boot out of my sump when I changed oil.
