Canon iPF 750/760 loosing cyan ink

Hello everyone. I have an issue with Canon iPF 760 . The same problem I had with other two iPF750 which I have not solved. But this time I need help with iPF 760. It looses cyan ink. Sometimes I can get it going pretty well by either running cleaning B and /or running p.h. alignment. After these procedures,
I can run nozzle test as many times as I want in a row . Prints perfect But as soon as I leave machine not running for at least 15/20 minutes , let alone overnight . Problems comes back. What I have done: Replaced left ( colour) ink station ( used one). Purge unit ( new one) print head ( new one) . Updated firmware. Adjusted heights of the carriage. Nozzle check positioning , ink is canon original.
Problem still there if nothing is done to it.. Any ideas? To me all is left is main board and new left ink station?. I came across this problem 3 times already. And have not found solution yet. Please help.

 

Hi,
Check Cyan Ink supply function : The suction pump of the purge unit operates together with the ink supply valve to supply ink to the printhead during the initial filling and ink level adjustment.
Check and remove bubbles, if any , in the ink supply system , from the ink station to the head.
Low probability to suspect mainboard or ink station - already changed, even with an used one...