There are lots of threads on this topic. I think
this is the most recent one, but it also notes that there are lots of others. The problem might involve "siphoning" but I think it's more likely "spillage" as water pushes through the vent tubes with movement of the tank during normal driving.
You can plug the overflow tubes with valves or stoppers but you've got to remember to remove them when using the pump. I just tested a check valve inserted in the system today, and I think that helps solve the problem of potential tank collapse, along with rubber stoppers in the overflow tubes to prevent over-pressurization.