General Motors V5 and V7 compressors have a control valve.  A control valve measures refrigerant and adjusts the angle of the swash plate, to which the pistons are connected, and controls the stroke of the pistons.  If high pressure is measured, the control valve adjusts to shorten the stroke of the pistons, so less pressure is produced.  If, upon installation of a new compressor, there seems to be no 'pumping' from the compressor, it may be necessary to remove the control valve and blow off excess oil.  The control valve may be sensing no pressure and presuming the system is emtpy of refrigerant.