Crank case pressure regulators are normally fitted to condensing unit systems with 1 or multiple evap's and electric defrost (hot or cool gas defrost would mean that the compressor is running and therefore the pressure would not increase to an unacceptable level). This is purely to limit the pressure in the compressor crankcase on start up. Once the pressures have equilised there should be NO pressure drop accross the valve. Another way of preventing excessive crankcase pressues is to fit a MOP Tex.