When you say tilt or aux has to be maxed out for the lift to work....
Does that mean max it out AND hold it there?
Or simply tilting the bucket all the way one direction then letting off and then the loader lift works?
Dont really see how it could be a cylinder, since it shouldnt matter what the tilt or aux is doing. IF it was a bad cylinder, its going to be bad all the time.
And since all other functions work, I wouldnt suspect the pump either.
Sounds like you have something going on in the valve itself. Something with the lift spool not responding right.