Abstract: A valve assembly which is suitable for backflow prevention in a stop cock having an inlet, a seat and an outlet. The valve assembly may be attachable to the stop cock and includes a valve body having at least one opening and also including a valve chamber and a valve seat. There is provided a balance tube in the valve body which communicates with the valve chamber which has one end engaged with the stop cock seat for sealing against the seat for prevention of flow of fluid from inlet to outlet, a piston located within the valve body and engageable with the valve seat, and biasing means operable to unseat the piston from the valve seat to enable the outlet to communicate with the valve body opening. In a further embodiment the valve assembly also includes a third seat in the handle and a valve member biased open by a spring against inlet pressure to allow atmosphere air to break a vacuum at the inlet.