A relief valve for low pressure operation includes a valve body (12) having an enlarged cylindrical bore (20) in alignment with an inlet port (16) and defining a valve chamber (22). An outlet port (18) opens laterally from the valve chamber (22). A seat insert (80) is received in the bore (20) and defines an axially facing raised seat (92) circumferentially about the inlet port (16) at a location closely adjacent the outlet port (18). A poppet valve member (50) with a truncated conical configuration and a maximum outer diameter only slightly less than the inner diameter of the enlarged bore (20) is mounted for engaging the seat (92) and blocking flow through the inlet (16). A biased stem (44) extends into the valve chamber (22) in axial alignment with the seat (92). The stem (44) has a cylindrical end portion which is recieved in a cylindrical opening (110) formed axially into the poppet valve member (50) on the end thereof opposite the seat insert (80). The cylindrical end portion and the cylindrical opening are sized to permit lateral shifting and alignment of the poppet valve member (50) relative to the seat (92) while preventing excessive and unwanted tilting of the poppet valve member (50) relative to the bore (20).