So the sender has an arm which on one end has a doohickey which floats on gasoline. As the level rises and falls this doohickey causes the arm to swing and move a piece of copper on the other end across a pole which is wrapped in copper wire. My understanding is that this mechanism generates the perceived fuel level. Right around the 3/4 mark a bit of this wire is catching the arm and stopping it from swinging further. I think a little sand paper will smooth and fix this.
This is something like what my car's sender looks like, the pole with copper wire is on the other side of the calibration adjuster, which may also be something you can use. Also note in the image the cover with the tabs, that was covering the bits you'll need to get at.
Hope this helps!
Oh and I almost forgot, in some cases all you have to do is work with the copper piece on the end of the arm so that it makes proper contact to send a fuel level signal. That did help the sender's accuracy for me.