Brother J4420DW - Yellow Color Problem - Event After Cleaning?

Hi, I've got a Brother J4420DW and have some issues with the yellow ink. I already did a cleaning with a cleaning fluid as described in this video: https://www.youtube.com/watch?v=zbqlxxPka-0 I did the cleaning for several minutes - it was a lot of yellow ink on the kitchen roll so I thought that everything would be OK after some test pages - but unfortunately the printer won't print the yellow ink. After some fails I tried to fill the ink channels with some yellow ink - after this procedure I was able to print out yellow colour but after some pages the yellow color got paler and paler until it turned to white. :-( It looks like that the printer doesn't pump the ink to the printer head - any ideas what I can do now? Btw. all the other colors are printed out correctly :-/?