Whether manually or power actuated, hydraulic car jacks have seals to contain the hydraulic fluid pressure. Seals are around the piston port on the cylinder, the fittings to the cylinder and between the hydraulic fluid supply and the cylinder. Manual hydraulic jacks are naturally simpler devices and require disassembly and a seal kit for replacing worn and failing seals. It's always best, however, whenever disassembling any hydraulic device to first drain the hydraulic fluid. Do not worry about storing or saving the fluid. When the repairs are complete add fresh fluid to the reservoir.
Torin T91203 12 Ton Hydraulic Bottle Jack
Large hydraulic car lifts for lifting entire vehicles are powered by electric pumps to generate fluid pressure. Pump repair involves seals, fittings and the electrical motor. If a pump malfunctions begin repairs by disconnecting the power supply. It's important the pump impeller be completed sealed. Check the seals around all shafts entering and exiting the pump. For checking electrical issues you will have to plug in the pump and use a voltmeter attached to the wire leads to check for proper power output.
Fittings and hoses
Fittings are high-pressure connections between the pump, hose and jack. With fittings and hoses the only possible repair is replacing the part. Also, if either the fitting or the hose fails and begins leaking, then both the hose and connected fittings must be replaced. Occasionally, you can cut a hose and attach a new fitting, but this usually shortens the hose to the point it loses its flexibility. Better to replace the hose than have it rupture quickly under unnecessary stress.
Powerbuilt 640912 All-In-One 3-Ton Bottle Jack with Jack Stand
If the cylinder or piston is damaged (hit by a vehicle or a large tool for example), the repair will probably involve replacing the unit. Trying to repair the metal components of a hydraulic cylinder and piston and have it still remain aligned and completely sealed is not cost efficient. Ordering a new mechanism and replacing the damaged unit will take less time and money.