The relief spring fits inside the cupped piston. If you install it with the spring bearing against the closed end, you'll have WAAAAY too much pressure, due to the spring being compressed too far. Be sure the valve slides freely in the bore. On the pumps I've assembled, it generally takes about 1/2" to 3/4" of squeeze on the spring to get the roll pin to install in the pump base. A firm push with the tip of a small screwdriver should unseat the valve and allow excess oil to flow back into the pan. Do you have the pump base installed in the right position? I think it's idiot-proof, but it's been a while since I had one apart. The output pressure should be able to bear on the end of the valve on its way to the oil passages in the block.
Jerry
