I know it's not a thirdgen, but at least it's a Chevy! I posted this at FSC as well, just trying to put the feelers out. The PS systems on my Z28 and my truck seem somewhat similar from what I can remember so maybe I can get some help. Here goes:

I replaced the PS pump on my 89 C1500 because it was starting to whine a bit. Since it was before a long road trip, I decided to be safe and swap it beforehand. Now I have very little if ANY Power Steering. Sitting at an idle with the tires on the ground gives me nothing. I have to rev the engine 3000+ to get even a little bit of power out of it. I did everything the normal way lifting the front end up and going lock to lock and getting all the air out. I thought it was a bad pump at first so I got another one and does the same darn thing! It looks like the fluid is flowing through the pump when I take the cap off the reservoir. All the O-Rings were replaced, the lines aren't kinked, I made darn sure to take the plugs out of the back when I took it out of the box. Any ideas?

 

I've had that problem before, usually a defective pump. One time I went through 5 pumps before I got a good one. At the shop I work at we use new GM pumps, I know they are pricey.
One thing you can try is remove the regulator spring & strech it out some, then reinstall it. Maybe there a new one out there.

 

I'm on my 2nd PS pump with the same result. What would stretching that spring do? I'm guessing you're referring to the one on the pressure side inside the pump itself?

 

I'm thinking that when the rebuild the pumps they reuse the spring.
I think they work like a fuel pressure regulator, the pump makes the volume while the spring keeps it closed, till theres enough pressure to open (overcome the spring) the regulator. If it opens too easily, then you'll have low pressure to the gear box. I don't know if streching the spring out will do anything, but it might put more pressure on the regulator piston. These pumps are really basic, I don't know why they(the rebuilders) have so much trouble with them.

 

UPDATE: I swapped back to the original pump because I figured the remanufactured pumps were to blame. That made it WORSE! Now I have ZERO power steering even when revving the engine. I double checked that the lines were clear, new o-rings AGAIN, nothing. Any ideas at all?