Browsing the forum I see nobody seems to want to know about your problem AJM. Maybe 'cos the site looks pretty 'orange' orientated at times.
Anyway, you could have been a little more forthcoming with information.
How old is the Blue Beast? How many hours? - 45 mins - warms up...? How hard do you work it in the 45 mins? How hot does it get? Objective or subjective analysis: Do you measure temps to determine heat? If so, where and with what? Hand, Thermometer, Digital Thermometer...?
1999----unkn hours---normal maneuvering----operating temp, tractor not overheating---
When the steering fades away, do any hydraulic functions fade as well? FEL & 3PH still work? --------------steering is only part malfunctioning
Regularly serviced? Last oil/filters changed when? serviced regularly ----- new filter installed has no effect
You see, it's hard to help you when you didn't help us much. Clairvoyance is not a diagnostic tool in my box.
Good luck with your pump change. Report back if you can.