Hi Alto,
Below I have tried to explain the phenomenon of Pump 2 without using symbolic terms, hope it will clear my point.
The function of Pump 2 is to fill the overhead tank; pump sucks the water from storage tank. In storage tank I use one sensor at bottom of tank for (Low level signal = Pin#5) this sensor must be activated (Giving -5 volts to Pin#5) for permission to run the Pump 2 in all conditions.
In overhead tank I am using two sensors, one in bottom of tank for Low Level signal and other is top of tank for Tank full signal. (Low Level signal is -5 volts to Pin# 6, Tank full signal is -5 volts to Pin# 7).
If the OH tank is empty, and storage tank is not giving Low Level input, it will run Pump #2 until the OH tank is full.
Pump #2 should turn on again when the OH tank becomes empty and storage tank is not giving Low Level input.
Note: My all digital inputs are negative 5 volts.