I made an assumption based on two things, that you said you can pull ink through the line, and that you didn't say you could see air in the line after doing so. Since apparently the bubble does return, I think that points to a leak at one end or the other, or possibly in the line itself though that is less likely. Does it form closer to the cartridge or to the damper? The answer might suggest which end is faulty. It could be one of the tiny o-ring seals.
About pulling ink through the damper, it's the same idea as pulling ink through the line, but leave the line attached and connect the syringe to the outlet port of the damper. Of course you have to remove the damper from the manifold (or whatever the right term is) to be able to do this.
I'd check the seals first.