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. When you find the source of the pressure leak, continue to the next step. If you cannot find evidence of a hydraulic leak, the problem is with the pressure release valve. Turn the valve all the way clockwise to fix the problem.
Release the pressure in the jack by turning the pressure release valve counter clockwise to repair a visible hydraulic fluid leak.
Open the fluid fill and turn the jack upside down to drain the hydraulic fluid out of the jack. Use a catch container such as an oil drain pan to minimise the mess.
Remove the retaining bolt on the part of the jack that is leaking using the adjustable wrench. Take care here, as some jacks incorporate springs below the pressure cylinder that you must not lose.
Pull the hydraulic ram out of the jack and locate the O-ring inside the cylinder from which the ram was pulled. You will see it just behind or ahead of the threads into which the retaining bolt was mounted. Pull the O-ring out of the cylinder using a pair of needle-nose pliers.
Match the old O-ring to a new O-ring and replace the old one. Wet the new O-ring with a small amount of new hydraulic fluid before inserting it into the cylinder.
Replace the hydraulic ram into the cylinder, taking care to not damage the new O-ring, and then replace the retaining bolt into the end of the ram housing. Refill the jack with new hydraulic fluid, and then check to make certain that the jack cylinder is properly seated by attempting to jack up your car. If the jack holds, the repair was successful. If not, then you may want to consider replacing the jack or having a professional repair agent inspect the jack.
Recycle hydraulic fluid at your local auto parts store.