Fuel pressure regulators typically fail through a tear of the diaphram inside them that then allows fuel to flow throught the vacuum line directly into the intake system. That vacuum line doesn't seem large enough to bleed off as much pressure as you feel you have lost.
Just because the pump is able to move fuel, doesn't mean it is able to move it at a sufficient volume to build sufficient pressure.
Maybe someone can post a procedure for testing fuel flow. Typically I've heard of it being taken off the return line from the regulator. Should see over a pint ever 15 seconds. But I don't know exactly.