RV Vacuum Breaker Check Valve,Camper RV Black Tank Flush Check Valve,Replace Leak Vacuum Breaker on Black Tank System Loud Noise & Wet Floor,Camper Backflow Preventer Improve Freshwater 571-VAC-CHK-A

So it's not a valve like you're thinking. It's a one way valve so nothing can come from the black tank back into your hose through the flush hookup. They start leaking a lottt, had one in my trailer go bad as well. Here's the replacement i bought