I have a flat tank with a big bung lid in the top, in the middle od that is a hozelock male connector which I fill the tank from, if I forget to disconnect then the vacuum buildup will stop the pump eventually as it can't suck any harder.
However if its an air lock they can be difficult to shift, you can think you've tried but it can still be there. This may not help you but I also have a backpack and I connect that up and pump water back through the whole vanmount. Never failed yet.