Your symptom is most often a result of failed piston seals in the steering cylinder. It's rarely the steering valve, and never the pump. If it has only one cylinder it must be 2 WD? Having replaced the cylinder (among other things) SHOULD have corrected the problem. I would suggest you revisit the cylinder as the cause. You can experiment by either fully extending or retracting the rod, loosening the fitting on whichever end of the cylinder the piston is on at that point. Start the engine and steer in that direction and see what happens. If the piston is doing its job the steering hydraulics will load up, go over relief, and very little oil will show up at the loose port. This won't solve your problem, but it may indicate where to go next.
If this is a "new" cylinder it shouldn't be a problem, but who knows? With used steering cylinders the center of the stroke is where the seal leakage is most pronounced. The piston spends most of its life in the center portion of the barrel so that's where the barrel's inner surface gets the most wear and polish. Often even marginal piston seals will be more effective at the ends of the barrel so that must be factored in.
