[karimwassef]

pistons require seals that I don't know how to design or maintain. They also take up the room for the tank and plunger to actuate. The vacuum achieves the same without mechanical devices. More mechanical parts = higher failure rate.

In turns of actuating the release, it's the same. A sensor opens a valve when the water is high enough.

Keep in mind that the cost of a mechanical implementation is many times that of a tank with a vacuum attached.