I'm talking about a breather that will let air back into the container that is being emptied. If no air can get back into the container (due to sealing the opening around the tube), to replace the volume of water that's coming out, you end up with a vacuum. Once the negative pressure is greater than the pump pressure, you get no flow of water. Unless of course you have a really strong pump, and you exceed the crush-ability index of the container...
Same principle on your boat fuel tank, and we all know what happens when you forget to open the tank breather... You spend the next 10 mins trying to work out why the engine stopped...
You OK, it might have been closer to 20 mins...