GPVs are designed for installation in the oil return line between the lowest point of the flooded evaporator and the compressor. By design the valves are kept open by a compressing spring so the oil-refrigerant mixture runs into the oil reservoir. Once filled, hot-gas pressure which is inserted by a connector on the cover, closes the GPV and concurrently increases the pressure on the reservoir via an overflow bore. This pressure is used to impel the collected oil back to the compressor and thus to drain the oil collection container.