With my system i am 100% positive that it will drain faster into the sump then the pump can return the water to the display. My pump is 1300gph at 1' head. I am at 9' of head and estimate roughly 400-500gph. The drain and return lines are same size (.75" tubing) and the drains syphon(overflow tube) is 1" PVC, the return flows from the 1" syphon pickup tube to a 3/4" barb that the hose slips over, downstairs to my sump were it is fed into a 1/2" OD (so the hose slips over it) pvc tube. The return pump has fittings for .75" or 1" fittings. I have a 1"-3/4" adapter so the 3/4" slips over the pump returns upstairs and is fitted to a 1/2" PCV tube that directs flow into my tank. There now is no ball valves on the return, but a ball valve on my pump to adjust some flow to be bled off into my sump if needed (currently it is closed). You are correct that the system did reach equalibrium, but for the wrong reasons. The only reason it is staying steady is because my return is sucking in some air because the water level is so low. I have tried refilling the display tank with more water, and it just returns to the same level and the sump fills with more water. The drain is def much faster then the return pump. It is possible because in fluid mechanics the formula for the pressure on the line is just Po+ p*G*H where Po is atmospheric pressure, p=density of the fluid, G is gravity, and H is height. If for some reason this is greater then the output of the pump with the same equal and oposite presusre (being the same height)then it will drain faster. Even if you turn the pump off, the syhpon will continue to drain at its given rate and is independant of the return pump. The goal is to hope that the pump and syphon match themselves with a equivilant flow, yes there pressures will be the same because there equal and oposite heights, but there will be diff head losses with certain valves, elbos, bends...etc because both the return and syhpon have diff PVC pieces..

I hope i made some sense on whats going on

My goal is to fine tune the system with the ball valvesafter i get the tom aquatics overflow box and hope that it balances out.