Likely just that fill nipple.
Is it a stealth system reg or one of the older ones? It is probably best to clean and relube the reg internals. You may have received dirty fill(s) and it it the debris and find dust that could be causing the leaky fill nipple.
Also, what kind of material was your replacement o-ring made of? Regular buna (the standard black stuff) doesn't hold up well when used in a spot such as the fill nipple. The filling process is pretty harsh on that o-rimg so a harder material is better suited for the job.
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