No power steering assistance!

I really need your help. It all started with a powers steering oil leak in a PS pump connector. Two days after I repair it the PS assistance just went away. I replaced the PS pump and it did not work. I replaced the PS pump once again and it only worked for two days.
After all this I replaced the rack and pinon and it only worked for five minutes! It's frustrating! What can be causing this nightmare?

Thanks

Is your new system leaking oil, or air pockets in the system?

No, there are no oil leaks nor air pockets. I forgot to tell you that when I just start the vehicle I have PS assitance only during one or two seconds of the first turn, and no PS assistance after that.

What doe's the pump sound like in the parked pos, turning the wheel to the left, or right?

The pump is very quiet, even when you completly turn the power steering to one side or the other.

See http://www.youtube.com/watch?v=5WPLowC6aZo

You coud still have air in the system. Jack the front end of the car up and cycle the steering wheel left and right.; Don't hold the wheel against the stop position. If this doesn't correct the problem put a hand vacuum souirce on the power steering resevoir and try cycling it back and forth with vacuum assist to the system.

I replaced the PS pump and It worked. Hopefully it will keep working in the next few days. The PS pump is slightly noisier than the previous one but it does not matter to me!
Thanks for the help, appreciate it.

PDTA: I live in Mexico, in Juarez. across EL Paso TX, so I have to say Gracias Amigos.
:)