Your pressure testing results are a perfect example of a leaking cylinder. It does not eliminate other problems you may have.
To show why it must be a cylinder consider the following:
Assume a cylinder with an area of 2.5 square inches and a ram cross section area of 1 inch. Since the ram always extends the entire length of the cylinder above the piston we have 2.5 sq in on the bottom and an effective 1.5 sq in on the top of the piston.
Raising the load requires 1000 PSI. 1000 PSI X w2.5 sq in results in the force required to lift 2500 pounds. There is no force being applied to the top of the cylinder so the pressure there is 0 PSI.
If everything works as designed the pressure on the bottom of the piston will remain at 1000 PSI and the top will remain at 0 PSI.
If we have a leak at the lowering port of the valve nothing happens because there is no pressure there.
If we have a leak on the raising port of the valve the pressure will remain at 1000 PSI but the cylinder will retract as the oil leaks out.
If we have a leak on the piston seals in the cylinder the pressures will equalize on top and bottom BUT because oil is leaking by the piston it has the effect of not having the piston there at all. Now the effective size of the piston is only the size of the ram, 1 sq in. We still have 2500 lbs pushing down on the ram but the piston is now only 1 sq in resulting in a pressure of 2500 PSI. As long as the valves are not leaking the cylinder will continue to hold the load but now at 2500 PSI rather than 1000 PSI.
As far as the loader drifting down, that indicates a leak through the valves. With a leak in a cylinder, a leak through either the raise port or the lower port will allow the loader to settle. You are also exposing the valves to 2.5 times the pressure the would have if the cylinder was not leaking.
I would repair the cylinder(s) first and then decide if there was enough problems to go into any valve problems.
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