To repair a hydraulic car jack, clean the jack before starting, remove the plug on the cylinder, and drain the hydraulic fluid. Then replace the fluid, and replace the rubber plug. Test the jack before using it to lift a vehicle.
When you are working on a hydraulic jack, it is imperative to ensure it is clear of debris and dirt, as any debris that gets into the cylinders can do damage to the device. Remove the rubber plug that is on the top of the cylinder to drain the fluid. If the plug does not come out easily, use a screwdriver to pry it off, and if there is damage, replace the plug.
To drain the jack, set up a bucket underneath it, and turn it so that the fluid can drain into the bucket. Look in the cylinder to make sure that there is no debris, and if there is, clean it out with compressed air. When all of the fluid is out of the jack, refill the cylinders with new fluid. Make sure the cylinder is full, wipe off the rubber plug, and put it back into place. The plug must be flat and fully sealed so that no air can leak into the cylinders. Test the jack several times with varying weights before using it to lift a vehicle.