ya like has been said before, the orifice lets the fuel pressure bleed off of the line/injector side back to the return and that lets the pressure drop to 0 with pump not running. the reasoning behind it is up for question? given the caddy type that doesn't use the bleed orifice, probably a carry over from when GM used the return type system on there mech fuel pumps with carbs or possibly worried about vapor lock with lower TBI 9-13 psi.