Are your jacks made by Dover/TKE? If they are the as I would second JcJmowery. With the added info that these check valves do go bad or it could be a internal seal leaking which you obviously won't see. Either way you would have to remove both pistons to get at either one and you might as well replace both if you go to the trouble of taking them apart.
I have mainly had issues with the Dover/TKE style telescopic cylinders but it is my understanding that other manufacturers use a similar design with the check valves as I believe my company replaced a set of check valves on an some OLS telescopic jacks as well once.
