Your loader has 40 years of wear and tear in the linkage that controls the spool in the travel valve. Start there. Slide underneath the tractor with a trouble light while you instruct a helper to operate the travel lever and foot pedal. Watch the spool movement as it slides in and out of the valve body. Too much play in the linkage will prevent the spool from moving all the way in and all the way out. You cannot get full speed unless the spool moves the full distance.
Use the search feature on this site and type in "banana plate" to bring up past discussions on this very issue.
Use the search feature on this site and type in "banana plate" to bring up past discussions on this very issue.