A compressor (20) has an unloading slide valve (100). The valve has a valve element (102) having a range between a first condition and a second condition, the second condition being unloaded relative to the first condition. A piston (124) is in a cylinder (128) and mechanically coupled to the valve element. A control valve (140) is coupled to a headspace (138) of the cylinder to selectively expose the headspace to a flu source (144), pressure of fluid in the headspace producing a force on the piston and valve element in a direction from the second condition toward the first condition. The compressor includes a pressure relief valve coupling (180) in the headspace to suction conditions to unload the compressor responsive to an overpressure.