I´m trying to understand the theory of operation of the oil return system in a parallel compressor rack. It´s my understanding that the oil is taken out of the refrigerant at the oil separator located in the discharge manifold. From there, whenever there is enough oil in the separator to open the float valve, the oil travels to the oil reservoir. From there, the oil flows to each one of the oil level controls, located at each compressor, and from there into the crankcases. What I cannot understand yet is how is the oil reservoir kept at a slighty higher pressure than the crankcases in order to keep the oil flowing. As far as I can see in this rack, whenever the oil separator float valve cracks open, the pressure in the whole oil return system must be that of the discharge line, not just a few pounds above the crankcase pressure. How is this high pressure not allowed to get into the crankcases and suction manifold ? Any comments welcomed.