I bought a 714Z a few months ago. I cleaned it and put it back together. The button on the spool sleeve moves up and down when it's not on the shaft but when I install it on the reel with a new spool shaft it doesn't move. Not sure exactly what the purpose of the button is.
If it is the button on top of the spool —it is a quick-release spool removal button.
With a new spool shaft — many times, there has been rust or crud inside of the sleeve that will need to be also cleaned up in order that the button release will work properly.
Plus a little drop or two of oil to free up the mechanism.
Best,
Fred