The service manual does a pretty good job of explaining how to disassemble the printer and remove the damper assembly. I would advise that you get the service software so you can evacuate the ink system then recharge it. I was having problems with the black channel after doing an ink swap. After I removed the damper assembly I was able to push cleaning fluid backward though the damper on all but the black. That may have been because it was completely clogged or because the way the swapping system works, it won't allow you to push fluid backward. The intake side of the assembly has pretty small ports and I couldn't find anything small enough to get a seal to push head cleaner from the intake side, so I flushed from the print head side of the damper.
You could pull your damper off and flush it like I did with mine, and assuming you can get fluid to flow through each port, you could then re-install it and recharge the system. You will use a lot of ink, and I had to recharge mine twice to get it to print a good head check pattern. You could try cleaning the print head while you have it apart.
Good Luck
David