Thomas is right, the OEM connectors were a known weak point. However, if that was the culprit it would likely stop the steering from working as well. The usual failure mode is that the internal valves close and dead head the pump, taking out the seal.
Disconnect and re-connect the couplers. Better yet replace both sets with domestic made units.
It is more likely that you have either dirt in the loader valve or the relief valve in the loader control valve has come loose. Take the crown nut off and see if the adjuster has backed all the way out. This is also a common issue with the OEM loader valves and it is an easy fix.
To really troubleshoot the issue you need a hydraulic pressure gauge mounted on a Tee with a set of quick connects to match the tractor. Then you can plug it in to the supply line and check the pressure, then swap it to the return line (back to the priority valve) and check the pressure there.
Priority valves do fail, but not as often as the loader control valve. Also, it is after the loader, so if it failed, it is more likely to stop the steering, it should have no effect on the loader which gets it's pressure directly from the pump.
With no pickup screen in the tank, is there a suction side filter? Some tractors had this feature. If so and it is restricting the flow to the pump, it could result in low pressure. No screen or filter means you are circulating dirt through the system and the loader valve has some pretty tight tolerances and dirt that will pass through the pump will plug the control valve spools.
As far as I know, there are no replacement parts available for the internals of the loader valve. Also there are no diagrams of it either. If you do decide to take it apart, work on a wide clean surface with a rag or towel. Take lots of pics to help put it back together.
Let us know if you have any questions and please keep us posted on your progress. It will help others later.