A valve for blocking migration of refrigerant in a system including a compressor having an outlet for the compressed refrigerant and an inlet for the refrigerant to be compressed, the valve having a housing with a inlet passage for receiving the compressed refrigerant and an outlet passage, and a piston serving as the valve element which has a through passage for providing communication between the housing inlet and outlet passages when the valve is to be open. A conduit has one end in communication with the compressor case interior, and preferably with the compressor discharge muffler, and the other end with the valve housing. The valve piston is normally biased to a position to close the communication between the housing inlet and outlet passage and the refrigerant pressure when the compressor starts and is operating is conveyed through the conduit to the valve housing to move the piston to a position to open the communication between the housing inlet and outlet passages.