Water flowing through vent of backflow preventer

TIA, You need to get a plumber certified in back flow preventer testing and repair. That is not a do it yourself project. THE BFPD is there to prevent contaminated water from flowing into the mains if a low pressure condition exists due to water main failure. Something like that hapened in Pittsburgh PA years ago and sucked pesticides into the water mains. A lot of people got sick, someone got sued for $millions and they had to flush the mains for weeks. This is a safety issue, and not just any plumber will do. Contact your water supplier for a list of certified plumbers.

Stretch

I probably has to be rebuilt. Inside are some seals and things that work to prevent backflow in case of a pressure drop. In most areas it must be done by and certified by someone that has the proper equipment to test it. In some areas they must be tested once or twice a year by a certified tester. Check your local code.

Backflow preventors have movable parts which are subject to fouling, wear and breakage. Although it is sometimes possible to buy repair kits, local governments tend to be particular about who can fiddle with them.

In my town, only certified plumbers can service them. Our Code requires me to have mine inspected and serviced every 5 years.

Mine flunked the standard test last year but he was able to lube some parts and certify on the second try.

Regards Old Al