In 2015, 2 months after we bought Viva (Hull #374) I realized that the black tank 90 valve was not working: vertical or horizontal same result.
The black tank cannot be closed ... embarrassing. 

A shipyard in Italy replaced the valve while the boat was in the water: I cleared the idea with Michel Charpentier. 

Finally, Bill Rouse suggested to grease the valve from under while the boat is on the hard.
It is very effective, I added it to the to the "On the hard check list"

Hope it helps 
Dominique Guenot 
sv Viva 
Papeete, French Polynesia