Bonnet or poppet went bad in it. Typical residential backflow, you remove the canopy and those two parts are inside of there. Typically on the side of the backflow there is a model number and size, go to somewhere like DBC irrigation in Greeley and they will have what you need to get it fixed. Other guess is that the device somehow split or has a crack in it, in that case you get to buy a new one. Home Depot will have what you need as well but they tend to be much more expensive. Good luck