As a test, disconnect the vent line connection local to the regulator. If hunting or other related problem stops, the problem is due to some type of vent line restriction. The focus should now turn to resolving the vent line problems. This could be anything from a bee's nest in the vent piping, to a rag or debris creating a blockage, to excessive lengths or vent piping which creates this restriction.

The most common cause of failure to lock-up is either debris lodged in the valve orifice preventing the valve from closing all the way, or, damage to the valve orifice such as a nick caused by welding slag or other foreign matter. Carefully disassemble the diaphragm case from the valve body local to the valve body by loosening the union or bolted flange assembly. Carefully lift the diaphragm off the valve body retaining the o-ring for when you reassemble the regulator. Inspect the valve orifice from above, the edge should be sharp and free of nicks. The plunger that presses against the valve orifice is attached to the diaphragm assembly and should also be free of damage. A slight round impression is typically present and this is where the orifice pushes into the plunger seat and seals.