With this model, it's easier to replace the whole damper assembly (AKA ink selector unit) as per the Epson repair manual. You will need to drain the ink lines (otherwise ink will flow out the end of the lines when selector unit or dampers disconnected). It's possible to replace the individual dampers in the ink selector (you will still need to drain the lines to avoid an ink mess), though there aren't instructions for doing this, you may be able to figure it out with help from people who have replaced individual dampers in the 3800/3880 model (similar damper/ink selector design), and some photos I attached. You can find replacement dampers online from China- they are dual dampers (each houses two color channels), and you will need a total of for your 7900/9900 printer.
The 3800/3880 damper replacement threads that may be helpful can be found here: http://www.inkjetmall.com/tech/showthread.php?512-Epson-3800-ink-selector-assembly-disassemble-!, and here: http://www.inkjetmall.com/tech/showthread.php?228-3800-LC-channel-completely-missing/page2
And here are a few photos from my experience working on our 7900
It would be great if you could share your experience to help others who want to do the same.
Best of luck~ Dana