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