With a loader on a 284, there is PRV in the loader valve, and a second one back in the 3PH control valve. How are you loading it with the 3PH? The 3PH will(should) disengage itself before it reaches full height to keep drom possibly damaging the piston seals. You really need to either put something on the 3PH that is way too heavy for it to lift, or you may be able to close the 3PH speed reg valve completely by screwing the round valve handle down in front of the seat, fully clockwise. This will prevent fluid from flowing into the 3PH cylinder and force it to pass thru the 3PH relief. Then you can get an accurate reading of how the 3PH releif is performing.
The steering system dosn't build full system hydraulic pressure as it can relieve directly back into the hydraulic reservoir, and the priority valve limits the flow into steering somewhat. At any rate, I have a gauge permenantly installed at my loader input, and only ever see maybe 500 PSY when the steering is at the end stops.
I think it is unlikley that the rest of the system is causing the loader problem. You MUST build pressure to perform hydraulic work, and the loader control valve must do this in order to send pressure to the loader cylinders, so the most likley issue is the relief in the loader not allowing pressure to be built. As ERJ mentioned, you need to get your valve "T"d off of the main line between pump and loader, so you can observe what pressure is developing on that main line when you command a loader movement.