Ok Carmie and pvtdonut...here is why the siphon break is on the return.
The return line is designed mostly, to be a closed pipe, that is, the water is pumped from the pump, straight to the outlet(s). The problem is that water has this nasty habit of following the path of least resistance and it likes itself, so it will follow itself down...see also, siphon. The easiest way to stop it is to interrupt the flow, either by plugging it off, or allowing it to suck in some air.
The manufacturer places a small hole in the 90 degree elbow of the return to act as a break for the siphon. They place it there because its almost the highest point in the system. Technically, the highest point is the top of the elbow, but the water would shoot up and out of the tank and you would be mad, therefore, they point it down, If they didn't do this, and you had those return hoses pointed down, the siphon continue until the water level drained down to the end of them to the point it finally sucked air. Those hoses are long and this could be MANY gallons of water. With that little hole in the elbow (above the water line) the siphon can never form... unless it gets plugged for some reason.
The cap idea is ok, until the power goes out and your not home to cover the pipe. The better thing to do is make sure your
sump can handle the extra water contained in the pipes. I would find the maximum point the
sump can be filled normally, yet still hold the extra, and mark it so I know the working limit.