According to my 1975 Chevrolet Factory Issued Service Manual for Chevelle, Camaro, Monte Carlo, Nova, and full size Chevrolet, it goes through a 4 plus page detailed explanation of the PS pump and the pressure relief valves and the 2 relief valve springs. Long story short, the small spring and ball control the absolute max pressure the system will generate. This only comes into play when steering system reaches the lock or when the front wheels are against a curb or buried in mud or sand (exact words from the manual). The max pressure can be increased by removing a small shim under the ball seat/retaining bolt shown in LATECH's picture. This may or may not solve a jerky felling when maneuvering like when tight parking or backing up a trailer with minimal vehicle movement. The larger spring acting on the spool valve is more of a flow control valve that modulates based on the demands on the system in almost all driving conditions, like driving down the road and turning into or out of a driveway. Usually adjusting the larger spring is all that is necessary to get rid of the jerky feeling. The large spring controls pressure by modulating flow in the 200-400 PSI area. But, I will agree there can be times when removing a shim on the small spring and ball will help. The spec according to the manual is 1250-1350PSI as adjusted by shimming. As you can see removing shims from the small spring and ball can have dramatic effects on max pressure and going overboard can cause an excessive system pressure that can rupture seals and hoses. That's why I go for the large spring adjustment first. I did this on a Street Rod 2 weeks ago that had the jerky problem and the problem went away. Customer was very happy and there was no increase in max pressure that could cause future parts failure.
In a situation where someone is using a Ford Mustang II Rack and Pinion powered by a GM PS Pump, shims have to be added to the small spring retaining bolt to reduce max system pressure. As said GM runs between 1250-1350 PSI. MII racks require a little more than half that amount. Also the large spring pressure needs to be reduced to avoid the over sensitive power assist complaint. There are also restrictors (where the high pressure hose threads into) which have a smaller orifice to help with an over sensitive issue. Street Rod set ups can be a challenge because of the wide range of tire sizes, wheel backspacing, front end weight, and steering wheel diameters.