Abstract: A valve (10) for draining liquid from a reservoir, for example, the reservoir (56) in the base of an evaporative cooler (50). The valve (10) comprises a housing (12) having a first inlet (14) for supplying a pressure transmitting fluid, for example water, to the housing and an outlet (18) for draining liquid from the reservoir. A plunger (22) is slidably received in the housing (12) and adapted to move from an open position in which the outlet (18) is open and liquid can drain out, and a closed position in which the plunger (22) closes the outlet (18) and the liquid is prevented from draining out. Water is supplied under pressure to the upper portion (32) of the housing via inlet (14) and a sealing ring (36) allows a build-up of pressure in the upper portion (32), sufficient to overcome the force of a biasing spring (24), to cause the plunger (22) to move from its open to its closed position.