A fluid metering system includes a housing having a piston chamber with first and second spaced ends, at least one housing inlet passage, and at least one housing outlet passage. The housing inlet and housing passages communicate with the piston chamber. A piston is disposed in the piston chamber for reciprocal movement between the first and second ends of the piston chamber to alternatingly define first and second volumes therein. At least one first slot, and at least one second slot are formed on the outer surface of the piston, and extend from respective ends of the piston toward the respectively opposite ends. The first slot has an opening that is in communication with the first volume, and the second slot has an opening in communication with the second volume. The piston is rotatable within the piston chamber to alternatingly bring one of the first or second slots in registration with the housing inlet passage, while simultaneously bringing the other of the first or second slots in registration with the housing outlet passage. As the piston reciprocates between the first and second ends of the chamber, fluid is alternatingly dispensed from the first and second volumes.