An unloader valve apparatus is provided for an air compressor. The unloader valve apparatus comprises a cylindrical piston having a longitudinal central axis and movable along the longitudinal central axis between a loaded position and an unloaded position. The unloader valve apparatus further comprises a stopper member pivotable about a pivot axis between a blocking position in which the stopper member blocks an unloader port when the piston is in the loaded position and an unblocking position in which the stopper member is not blocking the unloader port and is thereby allowing compressed air from the compressor to flow through the unloader port when the piston is in the unloaded position.