Abstract: A relieving valve for a fluid or liquid storage tank to permit release of pressure from inside the tank as necessary. A piston is movable within the valve to open the same to atmosphere as pressure builds up slowly therewithin. Upon occurrence of a sudden or rapid pressure rise in the tank, such as when the fluid therein surges against the valve, a snubber assembly in the valve is moved by said pressure rise to a partially closed position to impede the rate at which pressure can build within the valve and thus increase the time before which the valve opens to atmosphere.