wa8yxm wrote:
Ok two different types of connectors to the tanks.
Bingo. The two types also have different sealing surfaces. POL originally had metal-to-metal sealing, and you needed a wrench to get it tight enough. Later, there were o-rings were added which helped some.
The newer Acme/QCC fittings have an o-ring in a different place, designed in from the start, and were made to seal when hand-tight.
POLs are more prone to leaking. That the OP's tanks don't leak with a different fitting attached, or that a different tank didn't leak makes me suspect that he has a system which has been using metal-to-metal sealing. Over time, you have to tighten them more and more to get a seal.
Question to Squeezle - does it use one of the hand-tight fittings, or do you need a wrench to connect it?
(The alternative is you just need a new o-ring)