Do I need 1 or 2 valves at each bag? I would think one, that I hit the switch to open it (lowering the truck), then hit the switch again to close it (allowing air to fill the bag) So, therefore, I would need 4 valves TOTAL (one at each corner). Right...or wrong? I read somewhere I need "Fill" and "Dump" valves..but don't quite understand that...
