After initially filling the system, have you checked the level? It should've dropped and a lot of bubbles will appear in the steering pump reservoir.
Wait until the bubbles disappear (10 mins), start it for 1 minute, turn your steering stop-to-stop, check for bubbles and top up the fluid. Again, wait for some time, if there are any bubbles until they all go away.
You have your clue right there with the fluid level constantly being close to full in the reservoir.
Replace your high pressure line!!!.
Its most likely internally collapsing under pressure, if you have fluid flow you will lower the level of the reservoir as the air pockets in the system get pushed to the top as you turn the steering lock to lock.
Since your saying the reservoir is always close the top your not getting any or very little fluid flow.
Just a thought i had, listen to the power steering pump, when you start up do you hear it laboring like when you would turn and hold the steering all the way to lock even though its in the center or anywhere in between but not at wheel lock?.
Another thought, pull the high pressure line off of the steering gear and check the one way valve on the steering gear, just poke it with a screwdriver and see if it has spring tension and returns to a closed position, also check that your O-ring isn't off the fitting on the hose and blocking the fluid flow.