I have got the same problem with a MFCJ6720 printer. [br]
I am currently printing 20 pages of full yellow* with setting to "photo paper2 and it begins to become slowly better.[br]
I prefer not to rinse heads because it's clogging the excess ink sponge unnecessarily.
It rinsed about 1/3rd of my yellow cartridge. Luckily, I am usin cheap compatibles.[br]
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*Just create a big yellow rectangle using Powerpoint and print that slide.