You're in real trouble if I'm the best you can get for a JD expert. There are REAL JD experts here, just not as many or as often as other brands. Looking at jdparts. com, this is a gear pump, not the radial piston pump that is somewhat unique to JD.
The best I can tell you is to get a manual and start testing pressures. It also sounds like you might have a couple different problems going on at once. Does the engine stay running if you play with the steering correctly while starting it up? will it warm up and then run normally? usable with the steering loader and backhoe? or is this just sitting at this point and unusable? how long have you had it, and what's the history/progression of this problem? Engines are much easier than hydraulics...