Just a couple of thoughts as I never tried switching back and forth between OEM and non Epson inks.
1. Check your damper for yellow. You might have a bad damper, and if you do, you can clean pretty much forever and you will not prime it.
If your yellow ink damper is dry, you might try a replacement. The service manual is on-line on many download sites and shows the process
on damper replacement pretty well. Part are not expensive, but it's labor intensive.
2. Check for air in the line. Not as common in the older non-pressurized printers, but you have switched out ink so you may a pretty large
air gap. It's easy to see by a quick examination of the ink lines. The only way to remove it I know of is to keep cleaning.
3. Try a Super Strong Cleaning Cycle. This is not runnable from the printer. I believe Epson's name is SSCL. You run it from the printer control
panel. It's only available with the latest firmware for the 7600. With as many power cleans as you have done, it may not help. However it's
much stronger than a standard Power Clean. It was developed by Epson for the 7600/9600 class printers when Epson figured out that standard
power cleans will not work. Thus it was added as a feature in the printer control software. I used to have the steps to run it, but have gotten
rid of all my 7600 stuff. Epson should be able to give you the steps.
I still think it's your damper. A bad damper can easily kill all the ink to the head as the damper is where the ink is basically primed for the head.
Hope you get it cleared.
Paul