If you use the latest Jetronic duel acting valve you can simply set it to cut off all air to the system after about 5 seconds and maintain it in the tank. This will only not work if the leak is at the valve, the tank or the fill check valve. The most likely of these is the fill check valve and it is quite simple and cheap to use 2 of these in series if you wish or simply put a festo stop end over the fill pipe after you have filled the system.

Deno