Abstract: Apparatus for diverting backflow includes a housing having first and second pressure openings for receiving fluid signals representative of first and second pressures. The housing has a gas inlet and a gas outlet and is formed with a gas transmission passage for carrying gas from the gas inlet to the gas outlet. The housing has a vent passage communicating with the gas passage and the region outside the vent passage for carrying fluid from the gas passage to the region outside said housing. Valve structure movable in the housing is responsive to the difference in pressures at the first and second pressure openings for blocking fluid flow between the gas passage and the vent passage when the pressure difference is greater than a predetermined value and allowing fluid flow from the gas passage to the vent passage when the pressure difference is below a predetermined value.