Black Flush connection doesn't work.

[ole dude]

Is the connector loose? Should not leak even though flussher may not work. I recon it could be stopped up, just work your way step by step to the tank looking for blockage, I wouldn't imagine a one way valve to keep the water from flowing toward the tank. Most units have a back flow preventer or vacum breaker to help prevent the black water from flowing the other way.