Bathroom flood - What did I do wrong?

dougrainer wrote:
Your problem is the Vacuum Check valve for the Black Tank flush system. The OEM's install this check valve usually in the Lavy sink area. The valve is faulty and needs replaced. Connect the Black flush again. open the Lavy sink doors and have someone turn the water on and observe if you have water squirting out. That valve if leaking is the culprit. Doug

Tony Lee wrote:
I would check the black tank flush system. Mine goes from the hose inlet, up in the back of the vanity and then back down into the black tank. There is a vacuum breaker at the top of the loop. (this is nothing to do with the drain system from the vanity bowl.)

One of the fittings was loose and when I turning on the black flush, had water spraying up inside and running out through the floor openings behind the water panel

Easy to check - take all the drawers out and have someone watch as you turn the black flush on.