Model 9BD backflow preventer for vending machine water supply lines prevents backflow of carbon dioxide gas and carbonated water into the water supply system to vending machines, thus eliminating the hazardous reaction of carbon dioxide with copper tubing. Its design features include a dual check valve to assure positive, dependable seating protection. For trouble-free longevity, it is also equipped with a ball check valve which is a third-check member, its main function being to prevent backpressure on each pump cycle from unnecessarily acting directly on the check members. Instant check valve response prevents unnecessary vent discharge during pump "off cycle". Vent discharges C02 gas to atmosphere in the event of fouling or malfunction of check No. 2 thereby safeguarding the potable water system from CO2 gas contamination. In line design minimizes pressure drop across the valve thereby assuring maximum pump performance.