Not sure how tough a Case cylinder is, I’ve attempted to rebuilt the cylinders on our JD loader and backhoe, they use packings instead of o-rings/backer rings, I just run em to our local hydraulic shop and let them mess with those blasted packings, it’s their bread and butter after all.
One thing you can try, reverse the hoses going to that cylinder, if it still does it most likely leaky seals in the cylinder.
Another way to tell, take the pin out of the rod end, take the hoses off, if you can shove the cylinder in and pull it back out easily then the packings are shot on the piston. Good seals/packings/o-rings should have a considerable amount of resistance on the rod, bad ones will have nil to none.
Far as the valve, plumb a 3000PSI gauge into each hose one at a time, activate the valve for that hose, if the pressure drops back off immediately you either have a bad load check or a scratched spool, if it’s anything like our Oliver and Whites with the variable displacement Vickers pumps the load checks can be replaced quite easily, I can do it in less than five minutes, if its a scratched spool your SOL. A new valve will be required, don’t goto Case unless you’ve recently hit the lottery, our JD loader has a standard 2 spool Prince valve on it that most likely came from Burdens Surplus Center. If you can’t lower the loader or dump the bucket with the engine off, then you have load checks, if the bucket dumps or the boom lowers with the engine off, you don’t have load checks.
Clear as mud?