canon i9900 sump access and cleaning

My i9900 prints the nozzle checks ok except the red bar is yellow. I
have changed ink carteidges and put in a new printhead but it still
prints the red as yellow. My printer is now several years old and I am
wondering if the ink sump has become full. Other than taking it to a
Canon service center, can I access the sump and fix it myself? There
is no error message or blinking orange lights to indicate a problem.
any help will be greatly appreciated.