Leaky Corny

[Blancaster]

I have a few used ball lock corny kegs, I have recently replaced all of the O-rings and new poppets. I checked for leaks and all was good. I filled 2 of them up and put CO2 to them. I noticed that one was not carbonating very well. I came home tonight and my nearly full 5lb CO2 tank was empty. I pulled the pressure release valves on each, one was still pressurized, the other was not... at all.
I switched tanks and hooked them both back up pressurized them took the gas disconnect off and then checked for leaks again. Still all good.
I then checked for leaks with the gas disconnects still attached. I find that the one tank leaks at the post....only when hooked up to CO2.
As I said they all have new O-rings. I also use keg lube. The disconnects are all brand new as well. I put on a used set of O-rings on that keg, lubed them back up and it seemed to stop the leak.... for now. Does anyone have any ideas why the gas post would leak with new o-ring, keg lube and new disconnects?
Due these 'universal' O-rings come in different sizes and thicknesses? I wonder if that is the problem?
Where is the best place to buy them online? I think the co-op may sell them... is it better to buy them there? Or can I go to home depot/lowes and get them?

Thanks
Brandon

[Chris Norrick]

The "Kegging 101" presentation I've done a few times had this PDF handout:

http://www.ovha.net/images/posters/OVHA_Kegging_101.pdf

It's got some McMaster part numbers listed at the end. I'm not sure if they are still valid part numbers. It's the gaskets I've always used. One purchase is almost a lifetime supply.

[Blancaster]

Thanks!! Much appreciated!!