On ours, the tank was about half full when the hoist was down. The air pressure is what actually does the work so it has to have enough reserve to not lose much pressure as the oil level falls when the hoist goes up.
I did the same thing with mine . Put it up with no load. Mine was a dual post but works the same. I welded an eye to the lower frame and pulled it down with a comealong. After it had been in use again for a while it got to where it would come downby itself albeit very slowly.