I had an idea in a dream where a pressure vessel had a buoyancy valve at the lowest point. The idea was that a ball would sit in a hole and water that may condense inside the vessel would lift the ball allowing the water to drain after which the pressure of the vessel would seal the ball back into place. That made me wonder about the possibility of a pressure based buoyancy valve and whether the physics were there.
What you’re describing sounds very much like a condensation drain in a steam system.
or a steam trap
What you have described already exists. (I hate it when I have a great idea and it’s already been done!)
In some applications, you don’t even need a positive pressure to reseal the valve, gravity is enough. The main weakness of this type of condensate valve is that they can be held open if there is any grit or dirt that prevents the ball making a good seal.
https://www.regulus.eu/en/condensate-trap-with-ball-float-incl-tube
I mean, isn’t that how submarines work?
water that may condense inside the vessel would lift the ball allowing the water to drain after which the pressure of the vessel would seal the ball back into place
In order for this to work, force exerted on the ball by the pressure differential between the inside and the outside must not exceed the maximum buoyant force on the ball. If the pressure differential exceeds that value, the ball will stay stuck to the bottom even when completely submerged.