I would like to ask if any trash got in the back flow discharge side? Maybe you did not get the back flow preventor put together right, you should have it inspected when finished with the repair. When I take apart the backflow to make the same repair you mention, I also open the water side and let any trash get out of the device. Is the check valve below the poppit and bonnet kit O.K.? Even though you can stop the leak in the vacuum breaker, it may not be serviceable due to the check valve having trash in it or being damaged or old.