You might try this to positively check out your cylinders. A hydraulic hand pump will pump up to 10.000 psi. You don't need to pump it up that high, but it is there if you want it. You will need to add a 10,000 psi gage to the line going to the cylinders. Hook up at one of the ports and pump up to the cylinder pressure, usually around 2500 to 3000 psi. If hydraulic fluid comes out of the other port or opening on cylinder, it is bypassing, fluid slipping past the o-rings or seals. Then try the other port and do the same thing. This will give you a feel of your cylinders ability to operate as advertised.
As far as the valve, you could probably test that to. but understand this. O-rings go bad or not perform as they should, and certain valves have certain tolerances, and some valves leak more than others. In military mil-spec items, each item is tested/inspected for specifications. In civilian world, maybe 1 out of a 100 might be inspected. As someone mentioned, there are pilot operated load checks, if you have them, and your cylinders are good, you will have more faith in your system. I see people working close to and under loaded buckets quite often.
I have often seen buckets left in the air, dump trucks in the lift position, and wondered why. Maybe someone here has an answer. This is a dangerous condition, if you have kids around. Maybe it is to keep the seals tight. Don't know.