Check valves are a false sense of security and can and will fail. A reef tank is similar to a waste water treatment plant where we avoid check valves like the plague, they have no place in saltwater aquaria.
As long as your overflow and returns are designed properly and you maintain sufficient freeboard in your sump to contain a minimal overflow you will never ghave need for one.
Same goes for drilled holes in the returns, they require cleaning and maintenance and still fail. There is no need for either in a well thought out system, period.