On 10 June, 01:34, Mike wrote:

With the ball valve coming straight off of the tank, I think I've
pretty much ruled out any of my connections,

There's a non-return from the compressor to the tank. If that leaks,
you can empty backwards through the compressor. Hard to spot, as
bubble fluid on the connector threads just isn't going to find it. I
once spotted this because the compressor oil sump was pressurising and
oil was weeping out of the filler hole!

If that's the remaining choice, try sticking a temporary ball valve in
there, then shutting it off and leaving it overnight (powered off).