power steering issue

ok i dont know if its the pump or the rack 96 legacy outback 2.5 .....power steering works great for the first 10-20 min of driving then it almost feels like ist binding up and i have to turn it back to strait no noise from the pump,belt is tight and fluid is full....if i bleed the system is works normal for about 10-15 miles before i have to pull over and bleed it again im not sure what to check next