Slow leak down of fuel pressure

Hi again , changed pressure regulator . Does
plenum have to be on and secured in order to
check the pressure correctly ? I disconnected the
return line and turned key on , pressure went up
and slowly down to about 30 psi and hold . At the
same time gas is leaking out of return line. I was
told that it should not leak. Again does it matter if
plenum is not on .  Thanks