A lubricant injector is provided including a body having a passageway, an inlet, an outlet, and an aperture for lubricant to flow from a measuring chamber into the passageway. A piston is slidable along the passageway. When the piston is in a retracted position, a discharge chamber is present in the passageway between the piston and the outlet. A biasing spring is disposed in the passageway for urging the piston towards the retracted position. The injector has a rest mode, wherein no pressurized lubricant is supplied to the inlet, and the piston is retracted. The injector has a pressurized mode wherein the piston is under pressure and is moved to its extended position to urge lubricant out the outlet. In the rest mode, the spring urges the piston towards its retracted position to create the discharge chamber and to open the aperture between the measuring chamber and the discharge chamber.