I had a failed jack that I took apart to see why it was not
maintaining the extended position. In my case the ramp would slowly
lower back to collapsed postition, which besides annoying, can be a
danger to the fuselage. Turns out that it was the synthetic plunger
on the main piston leaking. So even under pressure the oil would seep
past the plunger and the piston slowly looses position till it
eventually collapses. The culprit does not seem to be any of the
various ball-bearings in the base of the jack.

Anybody know if a replacement plunger could be obtained. I am sure
its much cheaper than a new jack.