You failed the quiz - oil is supposed to completely flood the rod side, the gas is in the remote chamber attached to the top of the cylinder. Gas is supposed to push oil and only oil through the transfer tube to force the bottom of the piston up, lifting the boom.
All of the seals including the gland seal have been replaced each time they've been apart - I think. The last time was just to replace the pistons so I can't be sure, but typically the guy takes everything apart, it is ultrasound cleaned, etc. I probably should be doing the work myself, but it is a little tricky. You need a couple of $500 sleeves to get the seals on the pistons and the pistons in the cylinder, that seal has to go by the oil port which requires some shielding, etc., so far I have avoided that. This time I am going to make the tooling to allow polishing the rod.
I wonder if I can get the rods ceramic coated? Some of them on my tractor are done that way and they hold up markedly better than chromed.
Pumping pressurizes the top of the piston, pushing the rod out. The gas (remotely) pressurizes the oil under the piston, pulling the rod in and lifting the boom. The rod seal is pressurized only to gas pressure (around 600 psi sitting at the dock), but always at the gas pressure; that is different than a normal vang where the rod seal sees full hydraulic pressure. But nearly the same at the dock, where the rod seal would normally see gas pressure x piston area ratio. It is dripping - about 1 drop a week? - sitting at the dock.
I was told I'd need two sleeves, one to get the polypak seal onto the piston, and another to get it into the cylinder. I was also told once pulled apart, you need a new polypak, as it gets scored coming across the elbow threads and nut threads. Piston would have to come off to get new rod seal on, or maybe you could work on the oil jaw side.
Gland specs from seal manufacturers usually assume concentricity, which often does not occur in reality. Their radial tolerances need to be met with the shaft and bore line to line on one side of the clearance.