Not true. The vacuum applied to the fuel pressure regulator insures that the pressure at the injector tip remains the same no matter what the manifold vacuum is. That enables the computer to use a consistent flow value in its fuel flow calculations. The consistent flow value keeps the mixture from being too rich or too lean at any given amount of throttle opening.
i8yoboty The regulator should hold vacuum steady for at least 30 seconds. A vacuum leak in the fuel pressure regulator sucks fuel if the leak is in the diaphram and air if the leak is in the metal case.