Before you take a cylinder off, you may want to clean and inspect the check valves. They have the similar symptoms of leak down, but unlike a bad cylinder, they have enhanced leaking under cracking pressure. I think that absent gross injury to the cylinder, probably more likely as the cause, but YMMV.
In the black valve bank box are three check valves, situated between each of the hydraulic lines. If you get dirt in a check valve, just barely moving the handle will cause the the cylinder to dump, rather than slowly increase.
I had this once; it was a simple fix- drop the FEL on something solid so you can release the pressure in the lines, and have room to work safely. Blow the area clean and use enough cleaner to get all of the dirt off. Then take a socket and open up the nut. As it comes unscrewed, oil will leak out, so be prepared. Again, have the area ultra clean, because the point here is to get dirt out, and there is a lip on the nut that collects dirt! Inside is a little gizmo (my recollection is that it is plastic). Clean it off, clean out the seat, and reassemble.
Call Terry if you want better details; I have a bad case of CRS on the finer details of this.
I haven't had any more troubles since I did that.
All the best,
Peter