Pressure relief valve malfunction?

[adrenolion]

Hey all!
I have an older ford 1720 with a liberty kpx-6/LW-6 that I'm trying to trouble shoot.
It started heating the hydraulic fluid with use and the fluid would look aerated after awhile. Then it basically stopped working all together.
I'm pretty sure that the pto pump is still good, after putting a shutoff in-line I can run it down and almost stall the engine.
So, I'm thinking pressure relief valve? I can't find a replacement for it or I would just replace and eliminate that possibility and move on to the next thing. (Valve body?)
Any and all help would be greatly appreciated! Parts numbers, exploded views, ideas, eliminating processes?
Thanks in advance

 

[adrenolion]

Pressure relief valve

 

[oldnslo]

Relief valves rarely fail closed they typically fail open.

Where did you have the shut off valve in relation to the relief valve when doing your test?

With shut off valve fully open What pressure do you see with all the valves in spring loaded home position?
What pressure do you see when operating a function and when that function dead heads?

 

[RayIN]

Here is a previous thread on a Ford 1720.

How to increase hyd. pressure is addressed here, but for a different model: Adjusting 7308 Pressure Relief Valve Setting - New Holland Tractors - Popular Compact Tractors

Ford1720 technical data.