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with preload the spring is pushing back with the same amount of force that caused the compression due to preloading. once overcome the spring has the original spring rate.
say a 10 lb / inch spring is preload two inches. you have to apply 20 lbs of force before the spring starts to deflect. but once the preload is overcome it still requires just 10 lbs to deflect the spring each inch. in the end, a total of 3 inches spring length change (2" preload, 1" deflection) is achieved with 30 lbs force (10 lbs / inch). 4 inches with 40 lbs, etc.
in the case of the oil piston and spring with washers you've preloaded the spring, but in the end the same amount of force is required to deflect the spring enough for the pump to bypass.
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