Idally check valves are used to maintain unidirectional flow.  In practice, because of size and weight restrictions, a check valve may produce some back pressure (pressure drop) under flow conditions and allow some backflow (leakage).  The specific requirement usually dictates how much the valve may depart from ideal performance.  The main criteria used in specifying a check valve are: fluid, operating pressure, operating flow and allowable pressure drop, back leakage.  See a few representative valves in this category then contact the factory for your custom check valve requirement.  