An air by-pass valve (1) has an air inlet (5), an air outlet (7), a relief port (11), and a relief valve (37) responsive to excessive air pressure for opening the relief port and dumping air to atmosphere. The relief valve comprises a valve member (39) mounted on a stem (23) and movable relative thereto. One side of the valve member is acted on by a spring (41) biasing the valve member to close the relief port. Air pressure acts on the opposite side of the valve member to move the valve member against the spring and open the relief port. The side of the valve member against which the spring acts is contoured so the spring force is unevenly applied to the valve member over the surface on which the spring acts. Uneven application of this force causes the valve member to cock against the stem when moved from its relief port closing position by excessive air pressure whereby air flow around the valve member is unbalanced thereby reducing relief valve hammer caused by harmonic vibrations induced by air flow around the relief valve member.