Kubota L4150 low power at loader and 3 pt

[mweller]

I have a mid 80s Kubota L4150 that has very little power at the 3pt and loader. Prior to me working on this it had gotten a new pump, hydro oil and filter. I made an adapter to bolt on the pump to check flow and pressure directly off of the pump, and I have good readings off of the pump (10gpm at 2500psi). Then I hooked my gauge up to the bottom port of the priority flow valve, which feeds the loader and 3pt, and I only get 5gpm at no load, and if I try to apply a load the flow and pressure drop to just about zero, then pressure slowly builds but tops out around 1600psi. According to the service manual I should replace the priority flow valve plug (cartridge), but I have talked to a couple Kubota dealers and they say they have never had one go bad, plus the plug is not available separately, only as a complete priority valve assy. So before I shell out $636 for a new one, does anyone have any suggestions or thoughts on this? The top port of the priority valve feeds the hydraulic shuttle drive, which works fine. I haven't taken it apart to check it, but the clutch locks tight when engaged and releases when disengaged.