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The absolute best defense is a properly designed overflow, returns and sump. No check valves at all are needed in almost all systems. Check valves are mechanical devices and will fail guaranteed.
Why did your sump overflow? Was it too full or were the returns too low in the tank or ???
Drilled holes fail also. The best bet and the only foolproof method is an air gap and sufficient room for backsiphonage in the sump.
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