I replaced my leaky rack and pinion in my 2000 Monte carlo LS recently along with the high pressure line. It was working fine for a week or so then I was having problems with hard left hand turning, but turns fine to the right. I thought the pump may have gone bad so I replaced it then I had no power steering either way. I figured I had gotten a bad rack since I bought it on eBay. Replaced it with one from a local parts store, same thing. I had flushed and bled the system after replacing all the parts and I still have nothing. I pulled the return line off and it is pumping through so I'm not sure what the hell is going on.

 

Check for leaks, as in it could be sucking air into the system.

 

Sometimes things go wrong in spite of our best efforts and this includes replacing one defective part with another assuming , "It is new and has to be good".
Those of us that have been burned with defective parts learned to get parts from a dealership (OEM).

There are a lot of good aftermarket parts out there and there are also a lot of bad ones too!
If price is the only consideration, consider the trouble with now having to return what you hoped was a good new part. The extra cost for the dealer part the first time is worth the cost , believe me! I does sound like you received some questionable quality parts.

 

Sounds like your rack is full of air. There may be a leak but it is full of air. How did you bleed the system?

 

At first I turned the steering wheel back and forth multiple times with the engine on and off. After that I put my mityvac over the reservoir, but since I don't have the correct attachment I didn't work very well