That’s way too much. Has to have air in the line or someone replaced the oil with water. Close the ball valve and load the car. If it still compresses that much there’s air in the line. If not, there’s air in the valve. Have you bled it with the jack’s fully extended? Is the feed line ran overhead? I’ve seen pipe runs that leave a high corner where a bleeder had to be put in the 90. If you bring the dead zone back in to a reasonable distance so the car relevels at less than 1/2”, can it at least just relevel to the floor and stop?