Maybe you can get hold an an ink cartridge resetter? And use cleaning fluid to refill your original cartridges?
That allowed me to extend the life of the cartridges enough to thoroughly test the machine, before declaring useless.
If your nozzle check remains "Virgin", like mine did, one of the thing I did try at one point was an Ink Eject.
Again, the command never completed successfully, but this is one of the thing I did experiment with.
Make sure, btw, that you have one or two extra spare Maintenance Tank.
At the stage you are, I also suggest taking the head apart to experiment with directly injecting cleaning fluid or mineralized water into each ink inport, to verify that fluid does go through the head. Mine produced a nice "curtain of fluid" for each channel. I kept infering that the head was still functional from this despite what everyone I contacted or spoke to kept telling me.
About the pump.
I kept suspecting the pump, if only because I had read here, from Joe at Laguna Services, that, according to his experience, the pumps on these 4900 printers were notorious to fail?
But the friendly technician who came in once demonstrated that the pump still worked. Maybe you will need to take apart the right hand side of the printer (worth it) to observe the functionning of the pump? If only to witness that fluid does drain through the little plastic tubes on the way to the Maintenance Tank? I remember watching as I issued Cleaning cycles that ink was flowing through the tubes. One manipulation I did not observe the technician do was how he manually "primed" the pump? As he explained, he manually rotated what looks like a beige plastic ring around the pump, that looks like it has been made to be turn by a human hand. AS he manually rotated that ring, he said he could see ink being displaced through.
So, don't give it up. One thing I did try, was to suck ink out of the Ink Assembly Unit. I am sure that works for you. But. So all your line appear full?