What's causing this?

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To Reprime an Epson PX nobody should remove the Ink Supply System... its a easy thing with the Adjustment Program: "Inital Charge" will drain 30% of FULL Cartridges into the WasteInk collection Box and most certainly it will flush out all air pockets

That will work only if you have access to the appropriate AdjProg.

There are a number of the Artisan/PX/TX models where no such access exists but the tip is appreciated and well worth knowing for those printers that do have it.