[Carl n Susan]

The only safety device I am aware of is the operator. You, as the operator, choose whether to run the flush system and the position of the valve (open or closed). And we know the operator can be undependable. Seriously, the stock flush system is totally independent of the gate valves. I personally run the flush system with the gate valve closed for 5-6 minutes. A normal campground only provides water at 5-6 gallons a minute. With a 45 gallon black tank, there is no chance of overfilling the tank. I do set a timer and a chair next to the flusher. I sit there so I don't get involved with something that will cause me to miss the timer. Also I listen to the sound of the flusher. If it should go under water, I pull the handle!