The two lines from the p/s pump are the pressurized line and the return line. One of the lines being clogged wouldn't effect the p/s steering from left to right (they may cause your engine bay to be covered in oil
).
Are you sure you have no power steering from left to right? Or is it something in the mechanical linkage dragging from one side to the other?
I would put the car on jacks high enough that the front wheels were free to turn, then with the car off turn the steering wheel from opposite lock to opposite lock. If you cannot feel what you are describing I would turn the car on and test again. I would only blame the p/s if the symptoms only appear while the engine is on.
If you do the above test and the power steering is definitely at fault I would suggest either you have a p/s pump that has an uncanny desire to only feed pressure while you're steering in one direction (which may be a problem with the belt) or you need to get a replacement steering box.
Source: I reseal/adjust steering boxes.