Problem with black tank flush

[billrector]

I installed a gate valve on the sewer connection. I start by opening the gate valve and then the black tank valve. Once complete, I close the gate valve and open the grey valve. It will partially fill the black tank. I then open the gate valve (with black and grey valves open). I the close the grey valve and rinse the black tank with the black tank valve closed and then opened after a few minutes. Rinse and repeat until perfectly clear!
https://www.amazon.com/Valterra-T58-...n5L&ref=plSrch