Abstract: A fluid mixing valve has a canister which defines a piston chamber divided into an upper additive subchamber and a lower carrier subchamber by a piston. A pressurized carrier liquid enters the carrier subchamber through an inlet and exits the carrier subchamber through a restriction, which produces a pressure differential across the piston to pressurize a supply of additive liquid held within the additive subchamber above the piston. The pressurized additive liquid is injected past an adjustable metering valve into the flow stream of carrier liquid downstream of the restriction. As the additive liquid is depleted, the volume of the additive subchamber diminishes and the volume of the carrier subchamber increases as the piston moves upwardly. When the supply of additive liquid is exhausted, in one embodiment a screw plug is used to return the piston to a refill position, thereby enlarging the additive subchamber to prepare it to receive a new supply of additive liquid.