The vacuum breaker on the sewer flush is important. If you put a hose on the fitting, pressurize it, then lose pressure it's very possible to siphon water out of the black tank if the end of the hose is below the tank sprayer level. That's what the vacuum breaker prevent. Replace it with a brass one, but for the sake of public health, DON'T bypass it. You can easily contaminate your hose, and whatever water system it's connected to with the contents of the black tank.
It's there for a darn good reason!!!
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