Regardless of the shape of the end, the relief valve works on the cross-sectional area of the piston. A certain diameter of piston requires "X" amount of oil pressure to overcome the spring and regulate the pressure at that point. A restriction between the oil filter and the rest of the engine would simply cause the pump relief to operate at its set point, but would starve the engine for lubrication. The problem is almost certainly internal in the pressure relief system, so it's probably the wisest decision to replace the pump. Good call!
Jerry
