Well, I got a new maintenance tank, so the digital nanny is not holding me back. I unmounted the decompression pump and ran it to the side of unit, and it appears to run when it is supposed to, and if I take off the tube, I can feel a slight vacuum pulled when I put my finger over the fitting, so I assume it is running correctly. Much more vacuum and the tube would probably collapse and pinch off, so the light vacuum is probably correct. I never felt any positive pressure though, and the official description of the pump is "compresssion/decompression pump". But I have only seen it run in a single direction.
I've now taken most of the printer apart down to the smallest of parts and cannot see anything wrong. I can find no signs of ink draining or having been drained in the past during cleanings or photo to matte black ink changes. Is there a drain pump in the IS unit? I don't imagine they dump all the ink with the printhead, that would take forever.
I suppose there could still be a clog somewhere in the ink supply path, but there are so many tubes and manifolds, that it would be a nightmare to clean them all and put them back together again. Also, that might explain one or two colors dropping out, but all at once?
My best guess at this point is pump in IS unit (if it has one), clogged up manifold somewhere, or a driver on the control board has failed and is not driving a pump it should be driving. However, the control board is unlikely, as there appears to be feedback mechanisms everywhere that tell the controller if something is not moving correctly. My next step is to peer further into the IS unit and see if it has a pump, and if so, figure out a way to trick it into running so I can feel for vacuum or pressure. My worry is that the longer I have everything apart in my very dry office, the more likely everything ink related is going to dry up.
Any thoughts or opinions both welcome and appreciated.
Jim