I pump liquid soap out of drums with hoses and electric pumps, so this is easy for me to figure out. The end of the siphon must be lower than the source it is siphoning. You can siphon from A to B just fine, since B is lower than A, but connecting A to C then causes B to be higher, and connecting the two will in fact cause the higher pool to siphon backwards to the bottom one. When I need to get an exact weight on a scale, I use the electric motor to pump extra soap, and once I turn it off it starts to siphon backwards until I close the valve. Picture the sma'll pool being the container I am filling, using the pool at the bottom as a drum, A is an electric motor and the length between A and C has an on off valve in it. Unless the motors forcing up, gravity pushes it back down.