I think you've got a GREAT concept!
Take it one step further what about a type of one-way valve. The first thing that comes to my mind is a valve in the human heart. Blood flows one way but not the other direction. This would solve leakage and keep the pressure needed in the tank.
The one I'm picturing in my mind would look different than current tanks. Simply a hole with the one-way valve and the pressure valve located in a different part of the top of the tank. As a matter of fact, I think I've got an idea for a simplified valve to do this....
Wow, I think I'm going to try to make one....