DesignJet, Large Format Printers & Digital Press

I'm sure this issue has appeared before, but I've encountered an ongoing problem with my Designjet 800, not recognizing a new, cyan cartridge. It continues to show as being empty, and in need of replacement.

The problem began with the classic, 22:10 error messages, vaguely pointing to the ink supply system. Replacing the cyan cartridge with another new one, was to no avail.

I suspect that the point on the ink supply tubes, upper assembly, where the cartridges seat, may have become clogged, from non-use (I print in greyscale, 90% of the time). I flipped the two catches, and removed the assembly above the supply station.  The clear tubing, for each of the respective, four colors, is appropriately stained from ink... except the cyan tubing, which is uncolored, to my eye... suggesting that no ink is passing through it. 

Before gambling on ordering a new supply lines assembly, for my 42" machine, and possibly the supply station, itself, I'd like to ask if there is some way to flush and eliminate such clogs... solvents, a syringe to remove air and pull ink, forcibly...?

Any advice would be much appreciated!