Customer Questions and Answers for Washer Drain Pump Assembly by Whirlpool

You're not quite correct, the black flapper in the drain pump outlet is a "check Valve" or anti syphon device. It does not allow water to flush back into the washer, They can "stick" but that wouldn't normally cause a drain pump to fail. At present, the "flapper" is not available as a separate service part and you'll need to order the complete drain pump.