Bringing this old thread up again.
I just found out about this EVO system, or the manual calls it variable assist steering. Apparently it's junk, since GM is now offering a kit to bypass it.
I am not sure what all the system has in it, but where the rubber meets the road is the EVO valve on the back of the power steering pump. I always had trouble with my steering since I got my 99 Tahoe, and was getting ready to buy a new steering box. It just so happened the other day I developed a fluid leak on the back of the pump, and it was coming from this strange piece with a wire plugged into it.
Found out this is the EVO (Electronic Variable Orifice) valve is what was leaking, and it's part of a system that gives a lot of trouble. GM makes a kit to get rid of this valve, and just leave the rest of the system in place. I also found out you can take a standard older pump without this valve, unscrew the orifice fitting from the older pump, take the EVO out and install the older orifice fitting. You can then get a hose assembly from a 96 Tahoe that did not have this system, and hook it all back up.
I got a orifice fitting from a pump that came from a 86 ss Monte Carlo, and I also used the pressure line assembly from the ss also. I had to tweak the line a little bit to fit, but the fittings matched up. I can't believe the difference, my Tahoe now drives great, no more twitchy feel and wandering from the steering.
