To remove air from the internal ink system, usually a power clean cycle or two does the trick (depending on where the air is).
Dampers should be replaced every few years, and can not be successfully cleaned/reused. With the 4900, the dampers are come together as one “Ink Selector Unit”.
Imperfect nozzle checks can sometimes correct themselves with regular printing, but other factors including environmental conditions, mechanical age + health, etc… can lead to more difficult situations. For example, if you have clogged dampers, no amount of cleaning or flushing will get the printer working well again, until the dampers are replaced.
If letters aren’t printing, or fully printing on your nozzle check, then you have missing nozzles in your black channel. Windex and other ammonia cleaners are great for cleaning ink (on counters or the outside of a printer), but the fast drying agents make them NOT good for use directly on the print head or parts that touch the head, such as the capping station and wiper blade.
I hope this helps.