A refrigerant system includes a compressor with a discharge valve that temporarily restricts the compressor's outflow at startup so that the discharge pressure can quickly rise to force lubricant back to the compressor. The discharge valve also helps prevent an axial face of a rotor of a screw compressor from rubbing against a bearing housing axial face of the compressor's housing. When the system turns off under certain low ambient air temperature conditions, the valve closes to help prevent the system's evaporator from freezing up. The discharge valve includes a novel arrangement of a piston, a valve stem extending from the piston, and a valve plug pivotally attached to the valve stem. The pivotal connection ensures a positive seal between the valve plug and a valve seat. A somewhat restricted fluid passageway between the valve's discharge chamber and piston chamber helps prevent the valve from chattering as the valve opens.