Got your email- your lift is a GLC 030 or 040 depending on the lifting capacity & was built in the mid '70's. Can't tell from your pic the upright that's installed- another pic from the front is needed.

I can see in your pic the side rollers on the carriage which are in need of service to stop this side to side rocking motion. Some of these older Yales had a hoist cyl which actually wasn't bolted in per say- they were secured in the lift by sliders on the rear most channel of the upright. If these "sliders" are missing/faulty- all kinds of issues with the upright staging correctly, along w/ extreme looseness of the entire assy can occur. Combine these issues w/ the fact that the hoist chain goes under the lower chain sheaves on the hoist cyl which could in fact come off the chain sheaves which in turn will alllow the upright to fall unexpectedly from the chains now being "free"- this was the cause for my concern.

Send multiple pics to my email showing the chain path (with the upright raised to the limit) so I can see if this is the case.

As far as bleeding the hyd lines- if you remove & cap the lines at the hyd control valve- no bleeding will be necessary- you will however have to remove the side shift carriage from the stock carriage also.