No PageBlack function at all now

This summer when the crosshatch test pattern for my Canon MX870 became weak, I went online and found methods for cleaning the printhead. I taped the contact field and flushed each color port with water according to the YouTube instructions of several people. It seemed to work as the Pageblack came back to full function with a clean and proper test crosshatch and good document printing. After 2-3 month trip, I came back to use the machine, and no Pageblack at all. I deep cleaned multiple times and used a different tank to no avail.

I water-flushed the head again with no luck, and ordered a brand new head. It also refuses to function at all via the PageBlack feature. There must be something beyond the printhead that's not allowing PageBlack to spray onto the paper.

What could be haywire and how do I fix it? The MX870 has been my go-to home office printer for everything; it sends and receives super quality faxes relatively easy, prints decent pix not quite as good as my photo printer but adequate without using nearly as much ink, has a great operating panel, connects wirelessly to all but my iPad and any Macs that come home to roost, has front or adjustable rear choice of paper loading, and has a good ADF. I don't really want to replace the machine.