John,
On the 4 jack SMC leveling system that you have, there are three possible reasons for a jack to leak down. The first reason is very easy to check, and that is an external leak. If you do not have an external leak, either in a jack or hose, there is probably a 95% chance that your problem is the solenoid valve that controls the jack extension. That valve is probably leaking oil back into the pump reservoir. The third an most unlikely reason is a jack piston seal leaking fluid internally in the jack.
Gerald