hello all. i just recently got into brewing, actually my first batch is in the fermenter right now.
as far as leaks go, i am very experiences. my family owns a compressed gas and welding supply shop here in Blue Bell, PA.
i've heard every story of possible leaks.
everywhere there is a connection, you could be leaking. the o rings in the kegs could be cracked, thats where i would go first. no matter how tight the keg closes, a cracked o ring will leak. even the ones inside the connections on the kegs. get new connectors if you dont know how to rebuild the connections, they are cheap.
the connection on the valve on your co2 tank should never have tape or pipe dope on it. the seal is made on the face of the valve by the washer or o-ring. every time you take off your regulator throw out your washer and get a new one. we give them out when people get their tanks filled.
if there is an o ring in your regulator, do not use a washer. it will leak. if the o ring is chipped, get a new one, if you cant find one, pick it out and put a washer in it.
as far as regulators go, do not submerge them in water. if water gets inside the inlet, and then the cold co2 hits it, it will crack the diaphram. the bleed hole will start leaking. if its leaking, you need a new reg. they are too cheap to be rebuilt. its about $40-45 to get them rebuilt, and a new one is $45-50.
as far as leak checks go. use one for compressed gas. if you are worried about keeping it clean, they have ones that are clean for medical oxygen use.
they also make one that will make bubbles for gasses as thin as helium. thats what i use.
any questions pertaining to the gas side of things, fire away.