26 March update.
Here is the status on my 7880 saga: all 8 dampers have been changed, new flushing box, new cleaning assembly (complete unit).
I was still seeing incomplete nozzle checks and non-uniform coverage on the capping station. I have verified ink is flowing through the capping station (both tubes without any leaks) as well as the flushing box (and the flushing box is uniformly covered with ink).
I have installed flush filled cartridges on all 8 channels.
Also note prior I was running Pro inks. And I had remapped the LLK channel (slot 1) to run Pro MK ink.
I have run 1 initial fill and 3 power cleans (from the printer control panel). Here is the nozzle check. I am just starting to see pink flush only on slot 2, pro WM ink.
Also here are photos showing the ink lines after running the above init fill and power cleans. The two photos show the ink lines essentially as they exit the carts.
In this first photo, the left ink bay, the black tube at the upper left is slot 1, previously filled with Pro MK.
This next photo is the rear of the right ink bay. The three black ink lines from the top right to bottom left are slots 5, 6, and 7 respectively with Pro PK, CD, and WD inks.
Note that these are the four darkest ink types.
So here are my theories:
what I am seeing is ink staining in the four lines due to these being the four darkest inks. However, this would not account for the CD and WD being almost non-existent. It may account for MK in slot 1 as that nozzle check is getting better.
Damper nuts too tight causing ink restriction? However, I did previously see better nozzle checks.
Failing air pressure pump? However I am not getting any error service codes on the printer control panel.
Any ideas for next steps?