I have noticed when I pull the handle on the slide valve to dump my blackwater tank it seems to function a little strange. When I pull the valve all the way out, it flows for a few seconds then seems to almost stop. I can push the valve back in about 3/4 of the way, and the flow begins again. At this point the valve is only 1/4 open.

As part of a routine maintenance, the previous owner had both valves replaced in February of this year by Marathon in Texas. Since I have never had one of these valves apart, I don't really know how they look inside. I assume it is just a flat blade that moves out of the way and allows the flow. I don't understand why the flow would decrease when I pull the handle all the way out?

Can someone shed some light on how this might be happening? It's not really causing a problem, but I would like to understand what is going on.