641 Hydraulics "hiccuping"

I have 2 Ford 641 tractors. 641 number 1's hydraulics hiccup under load only. NO visible hydraulic oil leaking from unit when observed through access panel, especially from rear of cylinder. Entire unit removed from tractor. Overhaul kit installed. New piston ring and all new "O" rings where cylinder mounts. Cleaned and polished control valve, new "O" ring on unloader valve, pilot valve removed, cleaned and polished, new "O" ring installed. Pilot/backpressure valve seat removed, cleaned polished and new "O" ring installed. Relief valve in bottom of reservoir removed. Both springs are in tact, ball polished, new "O" rings and and reinstalled. New "O" rings on feed and return lines in lid. Still hiccups. 641 number 2's hydraulics work properly under load, NO hiccups. I swapped units from one tractor to the other, first tractor still hiccups with the hydraulic unit from tractor #2. So I removed the relief valve and swapped those, still hiccups. Put the original hydraulic unit from #1 tractor in #2, NO hiccups. This seems to signify the problem is in tractor #1 somewhere. But where? Only two hydraulic related items on the tractor itself are the pump and relief valve, which I swapped already from tractor #2. Pump has pressure, not sure if it's loosing it internally, will put a gauge on it tomorrow. Feed lines that run through transmission seem fine as I am not gaining in the trans or loosing from the hydraulic chamber. Any insight would be appreciated.

The cause of hiccups is a small leak between the control valve and the piston. The leak slowly allows the lift to drop a bit before the system senses that and makes a correction by lifting it a bit. Then just repeats over and over again. It will drive you nuts.

The issue is trying to find the leak. It look like you have addressed most of the possibilities.

One thing to check is the control valve. Remove the round PTO lever cover. Put a load on the 3pt lift and pick it up then turn the tractor off. Look in through the hole a the bottom of cylinder an look for drips. It should be very small, just a dribble. You might get lucky and see it. A mirror and small flashlight will really help.

Here is a picture of a leaking control valve. My opinion is that this is the usual cause of hiccups.

Thanks Kurt, I have performed these steps previously and can detect NO leaks from either the cylinder or anywhere else inside the unit while running
with a load on the arms and it's hiccuping. The telling thing in all of this is that the hydraulic unit on 641 tractor #2 was working perfectly and
when swapped over to 641 tractor #1, I still have hiccuping. That would seem to indicate the problem is with tractor #1 not the hydraulic unit
itself. So that leaves me with the pump, lines or "O" rings between the cover. I tried fatter "O" rings this morning, but no improvement. I am going
to try swapping the pumps next, as maybe I have some broken ball springs in the rotary pump and it can't hold the pressure. Your thoughts?

I may be wrong, but I wouldn't think that the pump could cause hiccuping. Once the arms are at a particular height, the pump is no longer in play until something farther down in the system leaks down some and the system opens the valve to auto correct the height so the pump can pump some more fluid into the cylinder.

I'm thinking the same thing Sean. The hydraulic system contains two basic loops.

Loop 1 is the pump loop. Hydraulic oil is drawn up into the pump where the pump circulates (pumps) it to the pressure relief valve. The relief valve allows pressure to build and dumps the excess oil back into the sump.

Loop 2 is the lift cylinder loop. The control valve diverts some under pressure oil from the pump loop into the cylinder to cause the lift. When you drop the load, the oil is released into the sump. At least that is the simple version. There is the unloader valve, check valve, back pressure and safety valve in that loop also that all cooperate to maintain height.

The only connection between the two loops is the control valve.

With hiccups, the leak is in the second loop.

Yes, I know I simplified this a lot. But I hope it educates a bit.

I'm still trying to digest all that you have tried. Something is confusing here and I can't put my finger on it.

I swapped pumps today from tractor #2 to tractor #1, the one that hiccups. Again, tractor #2's hydraulic system operated perfectly. So now everything
hydraulic that could be swapped from the non-hiccuping tractor to the one that had the hiccups has been moved and still we have the hiccuping. The
unknowns we are left with are the feed lines through the bottom of the transmission. If those are leaking, as is common with this model, then that
could be the source of the pressure loss. I see no noticeable fluid level changes in either the transmission or hydraulic chamber that is the norm
when the lines are leaking, so maybe the leak is so small or so recent it has not gotten to the point I can see the change. Looking inside the
transmission while running with the clutch depressed to stop the gear movement, I see no bubbles indicating a hydraulic leak. I think I may try
draining the transmission and see if that reveals a leak in the hydraulic lines. I'll advise on what I find. We all hate these types of problems.
Don't mind fixing something if we know what to fix, but this constant chasing is nerve racking. Thanks for everyone help so far.