Hydraulic jacks that leak down have broken or damaged seals and piston rings somewhere in the jack. One of the benefits of having a high-quality hydraulic floor jack is that the jack can be rebuilt at any time using O-rings available from any hardware store. To refill the hydraulic cylinder, make sure to use hydraulic fluid, as typical oils do not work properly for hydraulic jacks.
Locate the source of the leak by inspecting the floor jack. The point where it is leaking from will appear to be slightly moist. On most jacks, there are four possible points where the leak may be located: the main ram, the compression ram, the fluid fill cap or the pressure release valve.