Abstract: A pressure relief valve for monitoring pressure in relatively large diameter conductors containing relatively high fluid pressure. The valve having housing openings of equal diameter with respect to the conductor and a flow passageway provided with different diameter coaxial sleeves slidably receiving interconnected dual diameter pistons. The smaller diameter piston forms a flow passageway stop while the larger diameter piston generates a pressure differential with respect to the smaller diameter piston for moving both pistons toward a flow passageway open position in which a piston rod guides the pistons in their axial movement with respect to the sleeves and collapses a pressure collapsible pin supported by a pin cage connected with the valve body in axial alignment with the piston rod for opening the valve passageway and releasing fluid pressure from the conductor. The larger diameter piston is apertured to admit downstream fluid pressure to its low pressure side.