As far as I know there are two kinds of check valves. Spring loaded and flap. Spring loaded are closed even with zero flow. Flap require some minimum flow to operate. Spring type may be sensitive to the chemical makeup of the fluid since they probably have a metal spring. As with Dave, I'm not a fan of check valves. They restrict flow to some degree and are prone to problems such as gunk on the sealing surfaces. They do have their use in specific situations but I think they are over prescribed. Just my quarters worth.
Al