Pinch the return fuel return line. Does it leak down? If yes then pressurize it one more time, then also pinch the send line making sure the fuel pressure gauge is installed after the point where you pinched it off. This will isolate the fuel pressure gauge inside the loop with the injectors.
Does it bleed down still? If yes, then either the hose connecting the rails together or the injectors/injector orings are leaking.