For Febco 765-1 Repair Kit, 765 1" & 1-1/4" Model, 905-212 Bonnet Poppet Repair Kit & 905-052 Check Valve Assembly & Canopy for Febco 765-1 inch & 1-1/4 inch Backflow Preventer and Vacuum Breaker

The gaskets can dry out and tear or the spring sometimes goes bad. You can buy a repair kit from most of the box stores. Amazon also has them https://a.co/d/0c37NDCxCheck that the sizes match.

I have the same set up. I dissected mine before turning it on and it turns out one of the gaskets was totally dried out and wasn’t giving a good seal. Heres a link for the repair kit, for your reference - https://a.co/d/06uRghB6 It’s easy to service with basic hand tools.