To fill your 7600 with PiezoFlush to try cleaning it out after sitting unused for years, you should install the set of flush carts and do an Initial Fill cycle. If the Init Fill didn't fill the lines with flush, then you are likely dealing with a serious clog and need lots of cleaning. I recommend cleaning/replacing the capping station, flushing box, wiper blade, and cleaning the bottom of the print head.
You will need to replace the set of dampers, which will be filled with dried ink particles (don't replace the dampers until you have flushed the lines, or else the new dampers will get filled with crud from the old ink in the lines). If the dampers are clogged and preventing you from purging the ink lines, then you may need to manually flush the lines by disconnecting the lines from the dampers (with new dampers on hand to instal after flushing the lines), and drawing flush thru the lines using a syringe. Also, when the dampers are disconnected from the print head, you can place a folded paper towel under the head (as per our desktop pinter maintenance video), and use our PiezoFlush print head cleaning kit to flush the micro-channels of your print head, before connecting the new dampers.
I hope this helps, I wish you the best of luck cleaning and getting your 7600 printing again.