Power Steering Issue

[Ihavenocar]

While driving my car i noticed that the power steering would fail every now and then. Seeing how dangerous this could be (if it failed while making a turn) i changed the power steering pump. I figured it was the pump that was going so i bought another one (used). I installed the "new" pump and topped up the fluids. And cranked the wheel side to side in an attempt to bleed/purge the system. After doing this the power steering failure seemed to subside... but shortly after it failed again. Im wondering if there is a bleeding process that im doing wrong ? Im not leaking any fluids. Thanks in advance.