power steering question

Ok, quick general question for you guys. I have a friend who a couple months back replaced a power steering line on his vehicle but didn't bleed the system of air. His car became hard to turn, particularly when just getting going. He said a mechanic informed him that he probably just had air in his system so I told him to take the cap off his power steering reservoir, jack the front tires off the ground, and turn the steering wheel back and forth to release air from the system. I helped him do this. However, while it seemed there were plenty of air bubbles coming up the fluid levels never went down. I know this is probably a bad pump or rack given that the fluid levels are fine, just was curious as to why the fluid wouldn't go down, even with bubbles coming up and air seemingly leaving the system? Thanks.

 

Air is compressible. The air trapped in the system does not take up much volume because it has been compressed. Releasing that compressed volume won't result in much change in fluid level.

 

Thanks