Hi and welcome to JA. When you mean that the destroke is not working do you mean the pump is staying at full pressure or no pressure. Also does the steering work good? And the brakes? And by the low side of the remote do you mean the rear remotes?
Yes I do understand JD hydraulic systems. What is happening is you have a high pressure leak. And the reason you are cavitating is you are loosing your return flow. The easiest way to find a leak is let the tractor set over night the start it up and don't use any functions and let it run for about 15 minutes. then start feeling for a warm spot in the hydraulic system. If one hydraulic valve is warmer than the other then that would be your leak. Also does the 3 point settle fairly fast with the engine shut off?
I would suggest that you check for a heating valve next then.