I have now installed my new Paper Tower Air Supply line to replace the leaky one that was on the caster. I made a top connector from brass, giving it counterbores at either end to accept standard O-rings for sealing rather than relying on close metal-to-metal fit like the standard one does. It turns out that ¼″ soft copper tubing does indeed fit the tee under the table, so I used a length of that to rise from under the table to the top of the paper tower. The copper tubing and brass fitting were soldered together using plumbing solder.
After inserting the tube through the table into the supply tee, I put one O-ring, the new brass connector, and another O-ring onto the end of the paper clamp shaft, followed by the standard washer and nut. Everything was given a film of oil so the O-rings would not rub against a dry surface. The nut tightened against the end of the shaft, leaving the shaft free to rotate in the connector. There is a small back-and-forth rotation here as the machine cycles, and almost a half-turn when the paper clamp is raised for loading the ribbon so it is nice that is rotates smoothly.
But the best part is that it is airtight. The only air leakage left is around the leather packing that seals against the ribbon, and this is an order of magnitude less than the leak at the original connector.
I also want to add a spacing washer behind the collar on the paper clamp shaft just to the right of the connector. This will move the collar out to be flush with the shoulder of the shaft to give the O-ring better mechanical support, in theory increasing its lifetime. Testing with feeler gauges indicates that a washer 0.030″ thick would be just about right, so I will be cutting one out of brass shim stock. If the washer is too thick, the connector will start to bind on the shaft, so once I put in this spacer I’ll test the shaft freedom again.