A liquid additive injection pump has a hydraulic motor comprising a differential piston assembly reciprocable in a housing under the urging of a primary liquid into which an additive is to be injected. The piston assembly carries valves which are responsive to movement of the piston assembly to open and close to effect the transfer of a primary fluid charge from the pump motor to a discharge passage and to effect reciprocation of the piston assembly. The motor piston is connected to a central axially disposed longitudinally extending piston rod which extends into an additive injection pump cylinder and engages a floating additive injection pump piston having a stack of additive pump displacement control washers engaged therewith and disposed on the piston rod. A spring biased toggle type linkage snaps the motor valves into open and closed positions to provide positive movement of the valve members. Liquid additive is injected into the primary fluid stream at the outlet passage of the apparatus and downstream of the motor mechanism to prevent exposure of the pump actuating motor to the additive substance.