Sounds like the nut and piston came loose and are no longer on the rod.
I知 not sure how you came up with I diagnosis like that but if it were true the rod wouldn稚 retract.
Engine running, control forward, put a piece of wood near full extension, did a little compression then nothing.
Put control in neutral, pushed the ram back manually, ram will not go all the way back, maybe obstruction, vent hisses indicating vent open.
Control in reverse, engine running, ram pushes forward then when disconnect at rear end of cylinder fluid surges out while ram stays forward.
Checked tank, near capacity.
10w20 tractor hydro fluid, no foam.
I thought that also from your previous post. It implies that the piston does not return under hydraulic power, that you have to push it back manually.
Yep, nut came off or rod broke.
Sounds like the nut and piston came loose and are no longer on the rod.
I'm going with Barry. I think he nailed it, makes sense.