You said the hoe wasn't being used, is it hydraulically hooked up so you can check its functions? If the hoe functions and puts a load on the engine when cylinders are bottomed then the problem is probably not in the drive coupler or the main relief even though it is contained in the loader valve. If you take the main relief apart do not disturb the acorn nut settings, just remove the whole body and retrieve the spring and plunger internal to the loader valve. inspect the screen and clean it thoroughly the plunger has a very tiny orfice that if plugged will kill the whole hydraulic system, only once in all of them that I have worked on have I seen one that the screen came off and jammed the plunger open.
As far as the cylinders I would do a simple heat test on them, have someone run the machine at high idle and hold the hoses with the loader valve in raise mode and after a few minutes if you have a cylinder problem the hoses to the leaky cylinder will start to warm up. Sometimes your senses will tell you as much as a fancy test instrument which most do not have access to, also test your loader bucket cylinders the same way, also the hydra leveling cylinder.