"Jim Wilkins" wrote in message
...


I'd start with a semiautomatic system, separate manual shutoff
valves on the inlets to the small and large tanks and a check valve
that lets the large tank flow into the small one....
jsw

If you already have a check valve going into the small tank it doesn't
need a shutoff. The check valve will keep air in the small tank when
you open the valve to fill the large one.
jsw