[NoXid]

It is siphon flow to the print head. The piezo head ejecting ink is what ultimately moves the ink. But the capping station can suck ink through the head to during cleanings or when air might have been introduced into the system, like when changing carts. If the hose that provides suction to the capping station has a leak, then the printer may not be able to clear air from the lines and restore siphon flow. There is probably a separate hose to each segment of the capping station, so one for what would have been the K channels and one more for the color channels. A cracked "ink spike" thingy that the dampers are impaled upon could also cause a siphon break, but would affect just that color.

 

[NoXid]

Hmmm, so say you've got the head and dampers primed with ink, will it then print some ink for like a nozzle check? If not, I'd check the ribbon cable and its connections. With ink primed into the head and dampers, the head should be able to print some of that ink. Even if there were an obstruction or air in the lines, it could print the ink in the head. If it can't do that, the piezo channels aren't firing.

 

[NoXid]

This printer has a pressurized ink system (pump in the photo), so the answer is both.
View attachment 274846

During printing the printhead is generating vacuum, causing the one-way valves inside the dumpers to open and ink flows through.
During cleaning operation however, the capping station pump is doing the work, sucking ink through the nozzle plate.

Cool.

Where is the pressurized air introduced into the system? I imagine, but don't know, that it would be sent into the carts through a second port. So the carts themselves would not have a vent port open to atmosphere, as carts for smaller desktop printers do. I mention this because it almost sounds like a case of leaving the vent plugs in the carts ... except I presume that these wouldn't have vent plugs in the first place