Print color not functioning correctly

I have a MFC4420C that my son gave me when he got out of the navy in late 2003. It has been working great and has set idle for several months. I put new cartridges in it and ran it through the cleaning cycle 7 or 8 times and all colors except yellow are great. the yellow color on the little bars that print are great color and very distinct but not all are printing like the other colors. As a result when I try to print a color picture everything looks good except the yellow and it doesn't fill in all the color.
Any ideas?
Dennis