Most two port pumps have an internal by-pass mechanism of some kind to deal with a plugged fuel filter or other blockages. It almost seems that the internal by pass may have opened up permanently on your pump which is preventing it from developing pressure. If the pump is out of the car, use some fittings to attach the fuel pressure gauge directly to the outlet of the pump so that you are dead heading into the gauge. Power up the pump just briefly since if it works correctly there will be flow. You just need to power up for a second or two to see if it develops pressure. If the pump starts up; but, it does not develop any pressure then the internal by pass is stuck open or there is some other kind of internal mechanical failure.