When your hydraulic jack stops working then it's likely a seal has failed. This causes hydraulic fluid to leak out, the pressure to drop, and the jack to stop lifting. Replacing a seal can be a tall order and sometimes the replacement parts can cost more than a new jack. However, if you can isolate the leak to one seal in particular then you should be able to replace that seal for relatively little money.
Open the cap for the oil reservoir. With the bottle on its side, fill the jack until it is about 1/4-inch below the level of the hole. Replace the cap and wipe the outside clean with the cloth.
Place the jack under a vehicle and pump it up until there is weight on it. Continue to pump and watch the jack to find out where the fluid is coming from. It is likely to be coming either from around the piston at the top of the jack, around one of the screws in the bottom of the jack or around the piston under the handle.
Lower the jack, wipe it down and place it on a bench for further work.
Drain the oil from the jack by opening the oil reservoir and letting it into a tray. Stand the jack so the piston extends vertically and you'll find it looks like a bottle. At the top of this bottle there is a nut called the tank nut, through which the piston extends. Undo the tank nut using a pipe wrench. You may have to use a clamp or vice to hold the jack in place.
Remove the piston from the jack. Once the tank nut is undone the piston will slide out. There are several pieces to the piston, record their positions as you remove them so you can put them back the same way. The positions and order will depend on the jack you are repairing. Examine the seals around these pieces to see whether they are pinched, cracked or broken. Replace any that look worn.
Replace the pieces of the piston inside the jack and tighten the tank bolt.
Drain the oil from the jack by opening the oil reservoir and tipping the oil out.
Remove the screw that shows the leak and look carefully into the hole. There will be a ball or a pin if it is the release valve, or two balls and two springs if it is the overload valve. Carefully remove each of these pieces and make a note of the order in which you removed them.
Verify all the balls and springs are present and then examine the inside of the hole for the O-rings and washers creating the seal. Examine these for cracks, breaks or pinches. Replace anything in bad shape.
Put the balls and springs back in the hole in the order you removed them. Usually it goes small ball then small spring, then large ball and large spring. Depending on the jack, you may also have plates and guides to keep the balls and springs in place.
Once the balls and springs are back inside, replace the screw.
Drain the oil from the jack by removing the reservoir plug and letting the oil into a tray.
Remove the bolts securing the handle to the piston and to its pivot. There are usually three or four bolts. The number depends on the manufacturer.
Place the jack in a vice and pull out the pump piston, which is attached to the handle. It is not bolted in place but it may require a lot of force to remove. Put aside the piston and examine the O-rings and washers for any sign of pinching, cracking or breaking. All the seals are on the inside of the hole, in the body of the jack. Very few jacks have seals on the handle itself. Replace any worn seals.
Push the piston back down into the hole and replace the handle assembly, securing it with the same bolts you removed.
Replace the oil in the reservoir. Open the reservoir cap and fill it with hydraulic oil until it is about 1/4-inch below the level of the hole when the jack is on its side.
Remove any excess air from the jack by opening the release valve and vigorously pumping the handle. Close the release valve, then raise and lower the jack as you normally would. If there is still air in the system, the jack will not reach its full range of travel, so repeat the process until you get the full range of travel.
Wipe the jack with the cloth to clean the outside and lift the vehicle again. Determine whether there are any other leaks by examining the outside of the jack.
Make sure you are disassembling the jack in a very clean environment. Dust or grit inside the jack can cause it to fail. Clean all the parts before reinstalling them and soak any new, soft parts, such as plastic washers or rubber O-rings, in hydraulic oil to get a better seal.
Never work under a car supported by a jack alone. Use jack stands designed to support the weight. Hydraulic fluid is flammable. Keep it away from sources of ignition.