I don't think air-lock was your problem Craig, probably not low oil neither as you state no evidence of oil spill in the trailer...oil spill evidence just doesn't disappear...ever.
Those bottle jacks are a 'single acting" hyd ram only, there may have been a bit of air trapped in the bottom of the ram but it wouldn't interfere with operation at all, actually if all the valves in the jack were working correctly it should, although very slowly, still extend the ram. Air in a double acting hyd cylinder will cause "chattering" in it's operation until the air bleed itself from the ram thru a # of extensions/retractions.
I believe the check/ball valve in your jack just became gummed up over time & wouldn't close when it was supposed too. There are 3 valves in a bottle jack, the treaded 'relief valve" we operate from the outside, a ball check valve that works itself and the "free flow" valve that is opened when you operate the hand lever to full out position.
The free flow valve opening is simply an opening to allow oil to flow from its reservoir into the contained hyd pressure system. when you start the downward stroke of the handle it closes this valve and holds the stroke pressure within the system. There is an internal ball check valve between the handle stroke piston & the ram bottom that allows oil from the piston stroke to move one way only providing the treaded needle valve is closed, if this valve is open the oil simply returns to the oil reservoir without building pressure to extend the ram.
You will notice that when lifting something with a bottle jack that if the weight is much lighter than the jacks operating limit, the stroke is very easy but if your lifting very close to the limit, the stroke becomes very much harder, this is because with more weight pressure on the jack ram requires much more stroke pressure to open that check valve.
That is how the inexpensive "light duty" bottle jacks work but I have a 40 toner sitting in my shop that we used to lift D7 sized cats with to do the grouser bar welding i mentioned in another thread. That jack doesnt have a piston stroke to operate it, it has a rotary valve similar to a "Rotax engine" that will lift that 40 tons almost effortlessly...very slowly tho, maybe 10-15 thou lift on the ram to each stroke....that damn jack itself weighs probably 80 lbs.
