What a great name for a valve used in raw sewage! The joker valve would have to be really special to stop water from leaking back through when it passes Sh..., toilet paper, etc. Any tiny little piece and the valve isn't going to close completely. However the real purpose of that valve is to make the reciprocating pump work, not keep the water from flowing back. As long as most of the water passes through the valve and doesn't come back then the pump works fine. If the valve stays completely open water will just reciprocate back and forth in the hose along with the pump piston.
So you are going to get some water back, and about all you can do is pump vigorously to make most of the water in the hose go over the vented loop into the tank. You DO have a vented loop in the hose right?
Gary H. Lucas