PS pressure & feel

The power steering on my '90 LX is feeling kind of light and over-sensitive, and I'm wondering about reducing the pump pressure to firm it up. Gear seller AGR sells new rack & pinion units with "firm feel valving," but my understanding is that all manufacturers who have played with it have controlled assist by controlling the pump pressure. A company called Heidt's Hot Rod Shop sells a knob-adjustable PS pressure valve, but I was thinking of something more simple. Namely, I have seen the guts of some Ford/Mustang PS pumps, and have seen a spring inside, and was wondering if that controls the pressure. If so, wouldn't it be possible to start swapping springs around to get different pressures? If it's not the spring, is how is PS pressure controlled, and is it possible to modify it?

Do you currently have a 15:1 rack?

Yes. It was a rebuilt rack from Pep Boys, and was well worth it on my 170k mile car. But I'm trying to dial it in as much as I can. I also have Steeda caster camber plates, another worthwhile upgrade, and I'm running 245/50/16 Sumitomo HTR Z's on 16" Fittipaldi stars. I also have Motorsport B springs and Koni reds. But I'm given to endlessly ****ing with it, including doing my own driveway alignments with Longacre caster/camber and toe gauges (which by the way I have enjoyed much success with). I'm just wondering if there's a cheap and easy tweak to the PS pressure to try and suit my tastes. I know that the rack can be run without a pump at all, since that is the recommended entry-level application on my buddy's Factory Five Cobra, although I wouldn't try to run a Mustang without a pump in the street -- that Cobra has a 500 or 800 lb weight advantage, and the stock 15:1 5.0 power rack without the power assist is a bear at low speeds. Check out the Heidt web page for their pressure valve; it looks kind of cool. But if I can just rob a fistful of assorted PS pump springs at Pick-A-Part (if in fact the springs do regulate the pressure), I can probably approximate the same result on the cheap. Just wondering if the springs do in fact control PS pump pressure, or if it's something more complicated than that?