Since most corny's are reconditioned, aka used, they do not always seal by themselves. That is why people either put upwards to 30lbs of pressure on them after filling, use the McMaster Carr extra thick O rings, and or keg lube on them. Half of my kegs seem to seal fine with no assistance, where the other 3 take differring amounts of manipulating and pressurizing to get them to seal. Once sealed though, they seem to hold it fine. Better safe than sorry, nothing is worse than coming home to flat beer and an empty CO2 tank.