It stops leaking when you put your finger over the vent because you're stopping the flow of gas out of the pinhole leak in the diaphragm of the regulator. The vent is there to let air in and out of that side of the regulator so the diaphragm can move in and out to adjust gas pressure. All the gas should be on the sealed side of the system. Bottom line, if there's gas on the vent side, the regulator is bad.... And dangerous!!
Jim and Danna
2014 Kodiak Brown Ford Super Duty King Ranch
with 6.7L Diesel
2016 Coachmen Chaparral 360IBL