Hi @NackJich, I realize this is an older thread but in case you encounter it again, or for anyone else having similar issue, first thing to do is weigh the head. A full head weighs 60 grams. Anything less than that has air in it, with say 57g and lower being "too much" air. Actually I recommend everyone weigh their heads periodically regardless of how well they seem to be printing, as a good health check.
In your case I suspect the Y side of the head was basically empty. Then during the printer's maintenance cycles or during headclean cycles, my current working theory is that ink from the M side gets into the Y side through the nozzles while it's on the capping station. Since it's flowing through the nozzles only a relatively small amount of M ink gets siphoned into the Y side, which is why the Y side isn't completely M-coloured. And of course it eventually runs dry during your test print because the Y side is mostly air and the ink delivery system can't cope.
Replacing the head is of course one option, but the reason it works is because the new head has no air. If however your printer has air in the ink tubes, then that may not last long! Recommend purging any air from the ink tubes at the carriage and adding new head seals (LPS's video and their purge and seal kit worked for me on 2 printers).
https://www.youtube.com/watch?v=bsZ1Dqye_LoIt's also possible to also purge air from the heads and re-prime them with ink to work again (assuming the head isn't failed by now). I've posted recently on one way to do that.
Having examined the inside of these heads (see empty HP 70 MK-R photo attached) I'm not convinced that "leaks" between the 2 ink compartments is likely, although I know we're all inclined to speculate that could be happening. I assumed that was the case for years, too. But seeing how they are put together, I think it would actually require the head to be physically damaged with cracks in the plastic for ink to migrate between compartments. The more likely path of cross-contamination (path of least resistance) is from outside the head via the nozzles, as suggested above.
A good question is once a head has cross-contamination, is original print quality still achievable? I'm chipping away at that topic... I recently air-purged and ink-primed my Z3100 and heads and will be going thru print tests etc to find out. Certainly the attached MK-R photo shows some MK staining at the bottom of the R compartment. But whether that still results in "mixed" colours, and for how long, remains to be seen. I suspect it will work itself out fairly quickly since the volume of fresh ink now being delivered by the head greatly exceeds any residual from the earlier contamination. My plan is exercise the printer to see that the output looks stable, and then do fresh cal/profile.
I realize the "quick/easy fix" is just buy new heads (plus printer air purge), but that was much easier in the days long before HP started discontinuing products (HP 70 MK-R and R ink in particular) and rising costs + supply shortages for HP 70/73. And it's really frustrating (and silly) to be tossing a head with only a few prints from it because it got air into it. I'm fast becoming convinced that air really is the primary source of printhead woes/replacements, and by learning how to manage any air before it becomes a problem could make our heads last "practically forever".
Cheers
