I am doing the same exact setup except I am using the 800's, my parts will be here tomorrow and I am going to finish. I already removed the intake grate and grinded off the scupper and re-installed on the boat. I would bet when you remove the scupper it will stop water from coming into the system. With the scupper on when you drive the boat it is forcing water into the system and without the sprinkler valves the vented loops will not be able to hold back the pressure.

I will post up some pics when I am finished with the project.