The unload valve works without an O ring but if you do not fit the proper Ford O ring you will be in trouble, so it is good that you have fitted a correct one. I would still leave the ring off or fit the later unload valve that had a metal ring.
Did you check the pressure on the auxiliary service port?
Do you have the original auxiliary service plate with a flow control? If so, the flow control valve may be stuck. I have had to free one off only a month ago for a friend. It is a fairly easy job. Take the auxiliary plate off, on the back is a plug, held in by a roll pin, it has a screw into the gallery but do not worry about that yet. Drive the roll pin out then you have to get the plug out which may need you to drill and tap a small hole into the end, put in a small screw and lever the plug out. This is the official Ford way.
Once you have removed the plug, there is a spring, remove this. The valve will be stuck at the bottom of the drilling.
I made a piece of dowel rod and just tapped it in to the hollow rear of the valve itself. It needs to be tight as you are trying to rotate a stuck valve. A little easing oil of your choice will help. Once the valve will turn, try rotating the dowel between your hands as if you are grinding in valves. I have also used a battery drill on the end of the dowel to spin the valve and move it up and down the bore. You will find the screw mentioned earlier will stop the valve coming right out but once the valve is moving freely, you have no need to completely remove it.
The pump is the other area that your problem might be. You can get it out by removing the three mounting bolts and, with a wriggle, pull it off the two dowels that locate it. We used to do that without draining the oil, working by feel.
Once you have the pump out, before stripping down, mark the outer housing to ensure it goes back the right way round as you need it to be reassembled in the correct direction. Then remove the gear and D key. I have found that the gear has often come loose and sheared the D key. Then take the end plates off and check that the large O rings in both are in good condition and not blown. Replace rings and reassemble.
I used to love working on the hydraulic systems and trouble shooting on them when in the dealership.