This is an interesting problem.
-Definitely use POCB not PO checks. The check would probably chatter but you could live with it. Larger issue is that the check valves seal tight and don't have any pressure relief if the system heats up in say the hot sun when parked, with cylinder fully extended or retracted, depending on circuit. The oil expands, but is sealed tight, and pressure can build really high. A PO counterbalance is a spool device, not a ball check device, and is functions sort of like a relief valve if the cylinder pressure builds really high. POCB are usually set abouy 25% or so higher than the load pressure. There are different pilot ratios for different circuits to give stability but not create excess heat. For this cylinder load, basically any ratio between 1 to 3 would be fine. A 5 or 10 may chatter but would probably work if you got one free. Mount with hard connection to cylinder closed end. Preferably no hose or tube between, but this isn't a critical application like a crane. Not a pipe nipple if NPT. Use an actual hex hydraulic adaptor.
Personally, I'd just add a tube in tube and a pin since it gets used infrequently. If someone like Valley who uses it daily, I'd put the $$ for a POCB.
-The cylinder bleed down is valve spool leakage, not cylinder seals.
edit: mudd's linked video cylinder drift explains this well. i had not watched before. 7/3/17.
When the cylinder is loaded in the retract direction and the closed side is resisting the load, you could take the seals totally out and it should not drift. It would be a single acting ram like a snow plow, instead of a cylinder, and the pressure would be much higher, like two or three times normal. It will support the load by the much smaller rod area, not the full piston area. If pressure is high enough, the rod side seal could blow out, or other wise damage the tube, but if nothing fails the cylinder should not move.
Bleeding down over several weeks is as good as you will ever get with spool valve. I'd add the mechanical pin in stowed position, or the POCB.
-The interesting part is the motor lag. In one post you said several seconds, then 2-3 seconds, now fraction of a second. Several seconds is strange, but a second is not unusual. The fact it changed from before is of interest.
It would be helpful to have a pic of the flow control. I assume that was original, and was originally before the spool valve, true? Are there three ports? One from pump, one to the control valve, and one to tank? That is what I would expect, and is a bypass flow control, also called priority divider. Sends constant flow one way, bypasses the rest to tank. It throttles the bypass direction just enough to send the priority the right flow. It can be fairly efficient. With light loads.
A simple two ported flow control or orifice can give constant flow, but it throttles the flow downstream and forces any extra flow across relief valve at full system pressure. Fine for intermittent cylinder use, but a BIG heater when used in motor circuit. So I assume this used a three ported priority valve.
- Drain down?
Oil in the motor circuit is connected to tank through the control valve. A motor spool has P-A-B-T all connected together in center to prevent a hard stop on one side and cavitation of the motor on the back side. Oil in the hoses is theoretically connected to tank. However, assuming no circuit changes, the oil in hoses should not 'drain down'. Common misconception but since the system is supposedly sealed, it can't drain down. It works like an old style chicken waterer or hamster water bottle. Unless the height exceeds 30 feet of atmospheric presure, it won't drain back. (Technically, won't drain then either, it is cavitation. But a bunny trail.)
'Sealed' is the key. If the motor shaft seal leaks air into the motor case (which is connected to the A and B ports) it could drain down, but realistically the pressure drops across the internal check valves and across the seal, vs the just a few feet of height difference, I don't think this is it.
I suspect it is the conveyor belt compliance, or possibly belt going backwards slightly when parked. Long shot, but that could draw air through a shaft seal into the motor. It is possible a hose conmnection can draw air in but not leak oil out under pressure, but that is much more common on suction lines. Doubt that is the issue.
Try these:
1. Mark the belt or put a stick on it when parked. See if it moves backwards overnight.
2. When you start the engine, move the lift cylinder first before belt. Is there a slight delay in response? I.e. Is the delay occuring whichever function is operated first. Meaning it could be something in the suction or pump or flow divider, not the main control valve.
That should happen with every startup, noty just first time of the day.
Note: the priority divider will take a bit of time to respond, but usually it is the other way-It starts with a bit of a jerk too much, then settles down as the spool shifts and it controls flow.
3. Run engine at part throttle. Does the belt speed hold constant, down to some lower engine speed where it starts to droop? That indicates what type of divider it is.
If it delays several seconds, something is wrong. We shall keep on it.
If it takes a fraction of a second to start, I'd not worry about it. Just run it, but watch the motor shaft seal.
