my jack crapped out on my hoist as well, it would not lift and I did follow the directons of purging the air out of it, and yes it was full of oil,
started to act lilke it was low on oil and only pump up a few strokes and then stop, it was not really low but adding oil did not help. and then it stoped pumping all together, or only catch on a pump stroke ever now and then,
worked well when new,
as far as I can tell the check ball is not or will not seat, unless it is perfectly vertical. (would pump up ram in the shop in the vice) but will not pump at the tilt, mine was leaking out of the pump shaft/ram as well,
it is not spring loaded check only gravity, (at least mine did not have any spring looking thing,)
I took it apart and blew out the passages thinking there could be some dirt in it, and the ball was OK I could not find any thing that looked incorrect, replaced the copper washer in the bottom of the pump as the old was no in good shape, (the way the copper washer works it looks to lock the ball check in the port), but I just gave up and ordered a new one, hope this one lasts, they got me for $43 on the jack w/o shipping or tax,, or that is what was said,
I guess one could make a set of telescoping square tubes and a pin and set the angle or height and adjust before one uses the hoist, and just use the winch,