A few more details will help. I am not really familiar with that model, but more details may help others...
First, did it just start doing it all of the sudden, or has it been that way?
Second, is this only with an implement on, no implement, or all of the time??
Third, when it goes all of the way up, and you move the lever beyond 3/4 to full up, does it sound like it is still trying to raise and making the pump go into relief?? or is it just nothing??
Could be something as simple as a quadrant adjustment, or it could be something screwed up inside.