oil pressure relief valve theory

When I rebuilt the engine, the pressure relief valve never dropped out as it was supposed to. I did undo the large slotted plug in the bottom driver's side of the engine that I am sure is regularly confused for the oil drain plug (that has a hex head and is in the center of the engine). I have had decent pressure at speed and temp but low at idle and at running temp. My theory is that the pressure relief valve was stuck slightly open. The valve is a small piston about the size of the last digit of your pinky that sits on top of a spring. It is common practice to shim the spring with a washer to increase the pressure on the piston. This had been done in my engine but I do not think the valvle was moving. I pulled the valve out of a junked engine and it came out easily with a magnet on a stick tool available at autoparts stores. A tap fits into the bottom of the valve. This is what I used to pull the stuck valve out with. It was stuck and it did have some junk on it. I do not think it was moving based on the amount of pull it took to get it out even after I tried to wiggle it around with a screwdriver. I cleaned up the valve out of te wrecked engine, used the longer of the two springs and kept the shim washer in place. This morning I will test drive to work to see if it makes any difference with the idle oil pressure.