a pneumatic valve system for sealing off the flow of a liquid at the intersection of two pipes , one of the pipes being vertically disposed with respect to the other pipe . a sensing probe senses liquid at a pre - established height in the vertically disposed height which signals a control system to activate the pneumatic valve system . an expandable , inflatable bladder is positioned from the vertically disposed pipe into the second pipe and is inflated , thereby sealing off the flow of liquid past the bladder . when normal fluid flow is to return , the bladder is deflated and positioned back into its original position within the vertically disposed pipe .