From my limited understanding, if you leave the scupper in place, then while under way the scupper will force water into the system past the check valve and into the bag. Maybe not as fast as the pump would, but pump does not stop the water from moving either way and thus the need for a check valve or in the factory case, the sprinkler valve. With the mushroom, I think we are more concerned about the suction that is created under way and thus the check valve is installed. I would be interested in knowing how much time difference there is between 3/4" and 1" dedicated lines to the 750's and if it is worth the cost.