Did you replaced the Ink Selector Unit as a whole unit OR you replaced the individual dampers?
3 Power cleans is equal to an Initial Fill, if you replaced all the dampers OR the Ink Selector Unit, primed each channel and did regular cleaning cycles, that wouldn't have been enough to rid the system of air. Also, if you had primed the K channel, this would have only primed one of either PK or MK lines, depending on which was selected in the selector unit.
With the current K ink selected (I think it's PK from what I read) do 3 Power Cleaning Cycles to see if you can get a good nozzle check again for this position.
For the Capping Station, it's uncommon, but not unheard off for them to wear out, just like any other maintenance part in a printer. The function of the capping station is create a seal around the head for cleaning purposes, when it does this, in conjunction it moves the ink from the station to the maintenance tank. The part that normally wears out is the Wiper blade, which is connected to the Pump Assembly, and yes the wiper blade could be causing printing problems. If it can't clean the bottom of the print head properly, pigment will build up and cause blockage on the head.
Yes the Capping Station (Ie. Pump Assembly) can and will cause printing performance. But as I mentioned I would try another Power Cleaning cycle on which ever K you have already selected and see if you can get a better Nozzle Check, sometimes it takes a while to work the air out after opening up the ink supply system in these printers.