Power Steering Issue

I have recently had the power sterring pump replaced with a re-man part(as they claim there are no new pumps avaialble for this model) the steering is quiet for about 2 days then starts to sound like it's binding up - fluid is foamy and steering makes loud noises when turning. You can feel the tension in the wheel when you turn even the slightest corner. I have had the truck in 3x to have this same issue looked at - last time they said the O-ring was bad - had it replaced - it was perfect for two days and now is making the same noise. Generally starts to get worse the warmer the truck gets - any ideas?