I have no idea if the guy below knows what he is talking about but it's in the ballpark of your issue. I am sure you turned your steering wheel with and without the brakes applied when not moving. Also, I think I read you replaced your power steering pump. No leaks, right?
a shiet hydroboost full of gunk would not flow as well, and would cause a restriction which would allow more pressure to flow to the gear box, the new unit is nice and clean inside so the weak pump cant keep up with the flow of both. i would put a new pump on, there like 100 bucks from the dealer (do not buy a reman) and make sure you KEEP YOUR FACTORY PULLEY
also, IIRC the new pumps have a higher pressure regulator to address the known issue with poor turning while braking. so win win either way. install of just the pump is easy, just a few bolts and a couple 1 hard line 1 or 2 return line(s). when you get the new one, do not put fluid with stop leak in, it swells the seals for no reason.
https://www.duramaxforum.com/threads/power-steering-pump-sudden-hard-turning.739018/