Could possibly have a piece of debris stuck in the diaphragm or the diaphragm could be damaged.
Look into a rebuild kit if you feel comfortable doing so.
Turn the inlet 1/4 turn valve off and open the test valve petcock to relieve the pressure in the backflow preventer. The leak should stop. Close the test valve and then slowly open the inlet valve. There may be a gushing water out the top of the back flow preventer until the diaphragm sets. That might push any debris out. If not, then you need a rebuild kit or the back flow prevent has to be replaced.
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