Somewhere on here is an extensively discussed post about cylinder drift with valve closed which I cannot find. I found a similar thread from 2012 which discussed some of it. Here is my problem. I just resealed ALL cylinders on a Case 580D TLB. The problem cylinder is a stabilizer and was the second unit done, using a ring compressor and oiling the piston before installation. It is 90 miles on a service call from home and my wife and I finished right at dark and performed a quick operational and external leak test and all was good. No leaks, all performed correctly or so they appeared. The next day, the owner texted me that there were no leaks, but the stabilizer was "leaking internally". In 5 minutes or so, it will leak up from applied/extended, and down from retracted/stowed. with no external leakage. Down from retracted with a leaking piston seal is possible as it would create a vacuum bubble on the barrel side, but up from applied would require by hydrostatic principle, an external leak, which the owner assures me it isn't. I know this to be the case as I recently had a farm loader that would raise and HOLD with some sponginess after valve closing with cold oil, but after the oil warmed and thinned, it wouldn't raise at all. Upon lift cylinder disassembly as they were the only possible culprits, I found the piston seals to be completely gone and only the close tolerance of the pistons to the barrels sealed it enough to raise while cold. I had the offending cylinder barrel tied up for assembly and it went together perfectly so I don't think the piston seal was cut. I have ordered another seal kit and will go back on the 31st and investigate, but this doesn't make sense. A leaking valve is unlikely, but I don't see what else could allow this to happen.
Possible a piece of the old seals or something got stuck somewhere in the stabilizer valve unit? I don't see any load checks or port reliefs in that valve, but there are some plunger valves of unstated purpose, or it's possible the spool is not centering exactly? Obviously it had a problem before that the owner thought was just packing, but maybe not? Maybe the stabilizer valve had a worn spool or bad centering spring all along? Need to ask what this cylinder was doing before the reseal.
Leaking down in 5 minutes with no load seems pretty extreme. Before tearing it down again stick a valve in the line and close it and see if it holds to test the cylinder while isolated from the control. I find controls leaking past quite often on older equipment. Easy to do and you will know on which side of th valves the issue is.
The piston could be loose, wrong piston seal expander, or more likely the valve has problems
The owner says he would park it lifted off the wheels and it would stay up (have my doubts). I was thinking isolate the cylinder also. We resealed it as a whole machine reseal but it wasn't one giving any prior problems or so I am told.
https://www.youtube.com/watch?v=3E7dehBBvLw Or you can plug hose end with a gauge and watch for decay
Went back today and raised it up and sure enough, the cylinder would retract under load and fall under its own weight very quickly. I raised that corner with the backhoe, fully extended the cylinder and capped it, then set the tractor back on the stabilizer. It quickly settled a good bit then slowed and finally stopped with the wheel still off the ground. I fiddled with it some more and unintentionally hit the end of its stroke and to my surprise, the gland popped three turns out of the barrel with a bang. With that, I went ahead and took it apart and found the piston seal had sheared upon installation. The retract port is cut half straddling the barrel chamfer and with the gland against the piston on installation, it squeezed out into the port entry and was sheared without us seeing it as I turned the gland in. Poor engineering in my opinion! I installed the new new seal kit I took and made sure it entered correctly and all was well. The gland turned in and seated properly and if he doesn't hit the end of the stroke, maybe it won't pop out again. Thankfully he had me install and oil pressure gauge and weld up his loader bucket so it wasn't a complete lost warranty trip!
Thanks for the update and I'm glad it worked out for you. Always educational for all of is to hear what the problem turned out to be.