Acetylene Leak

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I have a pesky acetylene leak that I cannot find with any normal method.  If I crack the tank valve and then shut it off, the primary high pressure side holds fine but opening the secondary will introduce a leak that bleeds down in a few minutes.  I have replaced the hoses and the regulator is fairly new.  All the fittings were taped.  I have gone over every single inch of the circuit from tank to torch with soapy water and cannot find a leak that will blow a bubble.   The oxygen side is fine and will hold pressure overnight.  Anyone had one like this?
Reply:HW,What fittings are you taping?  Your hoses and your connection from your tank to your regulator should not be taped.Did you check the low pressure gauge?  You should be able to plug it's port.  I think you can also plug the outlet from the regulator.
Reply:on fittings like acet. and oxy, the threads aren't what seals it, it's the nipple on the fittingStangnetShop Full Of Stuff. Joey
Reply:I'll pull the hose and plug the regulator.  At least I'll know if the low pressure side is at fault.Get it hot and hit it hard...
Reply:Sounds like its leaking through the torch itself.The control valves on most torches seal onto a rubber O ring and if that is damaged or worn gas will leak into the nozzle even though the valve is turned tight off.You used to be able to buy service kits for most torches that included new O rings. But for "legal" reasons many manufacturers will not sell them anymore.I'm not very clever,But I can lift heavy things.