Thanks for the additional information.
From what you explain, PiezoFlush is flowing from all the carts, as all the lines are filled with pink PiezoFlush. If all the carts are down to 1/2-1" from the bottom, except the orange is still about 90% full, then something is restricting flow (though not completely, or PiezoFlush wouldn't be in the internal ink line). There is only one pressure pump in the printer, which controls all carts/channels, so the pump is not the issue.
Please reinstall the Epson orange cart and do a few paired cleaning cycles, then print a nozzle check to test if the cartridge is restricting flow, or if the issue is further down the line in the damper or head.
Keep me posted, thanks~ Dana