Have 54 NAA with piston pump. Last summer, lost hydraulic pressure while mowing. This happened after plowing the garden earlier that day. While plowing, had significant jerking when switching between draft and position control. Have always had a small amount of bobbing, maybe once every 30-45 sec. After plowing, mowed for quarter of an hour and noticed that the mower was dragging. Unhitched the mower and took the garage for repairs. Had 3 kids under 10, so the repairs took over a year. Zane suggested replacing the usual pins and turning the turnbuckle so it wouldn't bind. Did that, got distracted with learning to paint various tractor parts, re-wiring the tractor, fixing the steering, etc. Then recently replaced fluid (UTF) but nothing works. Now, I have no motion of lift arms at all.
The FO-19 manual says to connect a hose from the pump to the fill hole and make sure there's a good volume. I did that this morning and it looks good.
I also plumbed a 3000-PSI gauge to the accessory plate (FO-19, #12 in illustration FO72, labeled "Jack tapping plug"). With selector and control lever up (vertical), I get 0 PSI at idle and 1000 RPM.
I checked to make sure there was a good o-ring for the back-pressure valve to seat against. I checked the output of the back pressure valve with selector and control levers up but did not see turbulence. I looked for fluid spraying or dripping down from the lift cover. Did not see any. I checked the front wall for fluid but did not see any. I did spray some fluid around like an idiot when checking the pump volume (thumb on hose) so there were occasional drips but was like 1 drop in 5-10 seconds.
Have been reading this thread (below) which seems very similar. I am in the process of doing some of the homework... Going to feel for output from the back-pressure valve. The fluid level is low enough to see the system-relief valve but I'm not sure what it would look like if it were malfunctioning. Will look again. Also, I think I'll take off the lift cover (again) and make sure the unloading valve is moving.
Did this guy ever figure out what was wrong?