Since I haven't been using the last 4 channels, they're just filled with cmyk, I didn't even notice my black and yellow channels are out. I can see the ink and bubbles moving in the lines when doing head cleans, so I know the the head isn't clogged. I'm not sure how the pressurized systems work, my FP-125 isn't a pressurized system, but is there a chance that the leaking channel is getting extra pressure coming from those 2 missing channels?
I've read that if you're not using a channel it's best to leave it filled with cleaning solution. I just went through the process of switching my P600 over from the Epson inks to the DTG Inks and flushed all channels with cleaning solutions, using almost a half a liter in the process.
I'ver heard that other than White, Magenta is the other ink that gives people problems, so I wonder if you should clean that channel with cleaning solution. Frankly if you just cleaned the channels with a head cleaning I would take a step back and go through the process of cleaning all channels with cleaning solution.