I had the problem of my pump running dry when growing few plants in a large tray. The way I fixed it was by putting 2 liter coke bottles filled with yeast and sugar in the tray. That way when I water it takes less water to fill the tray. I also got the benefit of a slight increase in co2. Just an idea so you do not have to worry about the system running dry.
For you gallon milk jugs would work better. Even just with water if you do not like the co2 idea.